Notes on the Troubleshooting and Repair of
Small Gasoline Engines and Rotary Lawn Mowers

Version 2.38 (19-Jun-07)

Copyright © 1994-2007
Samuel M. Goldwasser
--- All Rights Reserved ---

For contact info, please see the
Sci.Electronics.Repair FAQ Email Links Page.

Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied:
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Table of Contents

  • Back to Small Engine Repair FAQ Table of Contents.


    Author and Copyright

    Author: Samuel M. Goldwasser

    For contact info, please see the Sci.Electronics.Repair FAQ Email Links Page.

    Copyright © 1994-2007
    All Rights Reserved

    Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied:

    1. This notice is included in its entirety at the beginning.
    2. There is no charge except to cover the costs of copying.


    Improper use, testing, or repair of gasoline powered equipment can result in explosion, fire, injuries including loss of limbs or worse, as well as total destruction of your spouse's prized flower bed.

    We will not be responsible for damage to equipment or property, your ego, or personal injury or worse that may result from the use of this material.

  • Back to Small Engine Repair FAQ Table of Contents.


    Can you see above the weeds?

    Most problems with gasoline powered rotary lawn mowers as well as other small engines have simple and inexpensive solutions. For example:

    This document addresses some of the most common ailments and provides detailed instructions for their diagnosis and cure. In addition, it provides detailed maintenance guidelines to prolong the life and happiness of your lawn mower(s) and other yard equipment.

    For electric lawn mower and other electric yard tool problems, refer to the document: Notes on the Troubleshooting and Repair of Small Household Appliances and Power Tools which includes chapters on basic electrical theory (relax, no rocket science) and electric motor testing and repair information.

    We will deal with problems the weekend gardener is likely to run into (sometimes, literally!) as well as semi-detailed overhaul instructions. One or more of the books listed in the section: References can be used to supplement this document and can provide much more detailed troubleshooting and repair procedures.

    Even if you don't know the difference between a carburetor and a crankshaft, you may still be able to do some of your own work. (Hint: if you drop one of these on your foot, the crankshaft will hurt a lot more!) We begin with an introduction to small engine technology and have more-or-less separate chapters on basic maintenance, intermediate troubleshooting and repairs, and more extensive overhaul procedures.

    I welcome comments, additions, hints, corrections, funny or other stories, etc. As the title implies, it is oriented toward the gasoline engine powered rotary lawn mower. However, much of the general information applies to a wide range of yard and shop equipment powered by small 2 and 4 stroke gasoline engines.


    Despite all the dire warnings (don't forget that the lawyers need to make a living!) it is possible to use, store, maintain, and repair gasoline powered equipment safely. Mostly, the necessary precautions are just common sense - for example, don't smoke around gasoline (well, don't smoke at all but that is not a part of the charter of this document!).

    The following represent the basic precautions to take when performing maintenance or service procedures on gasoline powered equipment and dealing with gasoline in general:

    Tools and supplies

    While you probably did not purchase your lawn mower specifically for the joys of repairing it, there is always a chance that despite all your precautions, the blade will strike a rock that just happened to grow out of the ground when you weren't looking. Therefore, it makes sense to be prepared.

    Basic servicing of small engines doesn't require a $500 tool caddy. However, some basic hand tools and other items will be needed.

  • Back to Small Engine Repair FAQ Table of Contents.

    Lawn Mower Basics and General Maintenance

    Gas, electric, or manual?

    Many people who have never used a gasoline engine powered piece of yard equipment are intimidated by all the warnings with respect to the explosive nature of gasoline. They then opt for an electric lawn mower instead of gas. For many, this is the correct choice. There are a different set of precautions to follow but they are fewer and seemingly less of a threat.

    Electric equipment is in some ways more environmentally friendly generating no pollution (though the electricity had to generated somehow). Once the equipment is unplugged, there is nothing to worry about with no gasoline to store. Little maintenance is needed and there is never any issue of disposing of used engine oil - since there is no engine oil. Electric equipment is also usually - though not always - somewhat quieter.

    The main disadvantage of line powered electric equipment is that it is tethered to an electric outlet by the power cord. This can become quite a nuisance after a short while. Battery powered equipment has tended to less powerful and more finicky to deal with than similar equipment powered from a line cord. And, electric mowers tend to be less powerful than similar equipment using a small gasoline engine.

    Where your yard is relatively small (say, less than 50 feet to an electric outlet from the farthest point), a corded mower may be a good choice. It will be less expensive than typical battery powered mowers and most gasoline powered mowers, and virtually maintenance-free. Just make sure you use a proper outdoor heavy duty extension cord - probably one size LARGER (lower AWG wire size number) than what the manufacturer recommends. This will assure minimal loss of voltage due to its resistance - and every bit of power you have available will help! A somewhat lighter duty outdoor cord can be used for the first few feet if that makes maneuvering the mower easier. The main thing to watch out for is accidentally cutting the cord by running over it. Mowing in a back-and-forth pattern while moving away from the outlet helps. If you do cut the cord - don't panic. At most, you will need to shorten it a couple of feet and install a new socket on the end of what is left AFTER pulling the plug! If the outlet is now dead, at most you have tripped the circuit breaker or GFCI, or blown the fuse. Of course if you make a habit of this, your cord could get to be quite short. :-)

    Battery powered yard equipment and power tools have improved greatly over the years. Some of the newer models are quite capable of cutting a modest size yard (e.g., 1/4 acre, manufacturers specifications may still be a bit optimistic) on one charge with ample power for moderately thick grass. But, there is quite a bit of variability in cutting performance and battery life so shopping around, consulting Consumer Reports, and making sure you get a return option if you are not satisfied are all well worth the effort - to save effort in the long run.

    It is interesting, however, that quite capable battery powered tractors for example, have been around for a long, long time.

    See the section: Comments on electric mowers for more information.

    Having said all that, the fact of the matter is that the vast majority of lawn mowers used for modest or larger lots are gasoline powered.

    Of course, if you have a postage stamp size or even a small suburban lot, a manual reel mower may be your best choice - and you get some good exercise as part of the deal as well.

    Also see the comments in the Chapter "Items of Interest" on electric and manual mowers.

    Types of lawn mowers

    Depending on the size of your lot, glowing conditions, and your energy level, one or more of the following options will fit you like a glove: The first two of these are described in more detail is subsequent sections of this document. For the last, well you should already know if that is appropriate for your life-style!

    Large mowers (those which carry you) may be of either the rotary or reel type, usually gasoline or diesel powered but some electrics have been produced. For information on riding mowers, lawn tractors, garden tractors and estate tractors (also known as compact diesel tractors), see the Small Tractor FAQ.

    Consumer Reports regularly provides reviews and ratings of most common types of lawn mowers. These articles are a good place to start as they include a great deal of the basic information needed to decide on the lawn mower type best suited for your property. They compare a selection of typical models based on features, safety, price, and their tests of performance and operator convenience. If you do not subscribe to Consumer Reports, your local library will likely have access to several years of back issues.

    For even more advice, see Consumer Reports Books' Yard and Garden Equipment Buying Guide. It is sold at bookstores and newsstands and is also available directly from Consumer Reports Books using the order form in the back of every issue of Consumer Reports.

    Reel mowers

    These may be manual or engine powered. A spinning set of sharp spiral cutting blades working against a fixed 'bedknife' snips off grass like a scissors. In principle, reel mowers can produce an exceptionally consistent manicured lawn. However, small reel mowers may have serious cutting height limitations and also result in a wavy uneven appearance.

    Rotary mowers

    These may be gasoline or electric (AC line or battery) powered. A spinning blade or blades whirls at the selected cutting height and lops off the tops of you grass by side impact. This is most definitely brutal treatment of your poor defenseless grass! The length of the cutting blade determines how much of a swath is cut on each pass - typically 18 to 24 inches for a walk-behind mower; up to 36 inches or more for a riding mower. Mowers with large swath widths may use two or more smaller blades instead.

    The uniformity, consistency, and just general appearance of a lawn mowed with a rotary lawn mower is not quite up to the standards of that of a professional reel mower. You will never get the perfect manicured look though some models may come close. However, the simplicity, lower cost, and need for less and more easily performed maintenance will generally overcome the desire for perfection unless your lawn is featured regularly in "Better Homes and Gardens" magazine.

    Gasoline powered rotary lawn mowers are by far the most common type used by homeowners and many professional landscapers as well. Most walk-behind mowers use a manual recoil (pull) starter though electric start is available on more sophisticated (and more expensive) models and generally standard on riding mowers and lawn tractors.

    The main disadvantages of a gasoline powered mower are the need to deal with the handling and storage of gasoline and routine engine maintenance at the end of the mowing season to assure easy starting next season and to prolong engine life. However, most of this is pretty straightforward. See the section: General preventive maintenance. Small gasoline engines also do contribute to air pollution but new mowers must meet more stringent EPA requirements as of September, 1996.

    Electric rotary mowers are also available in both plug-in and cordless (battery powered) versions. However, since gas mowers are generally more powerful and not limited by the length of a power cord or charge of a battery, they continue to dominate the market. Electric mowers are, of course, non-polluting but the electricity had to be generated somehow.

    Dead-man control

    All rotary mowers manufactured within the past 15 years or so must have a dead-man control to stop the blade quickly (within a couple of seconds) if the handle is released or the operator falls off of the seat of a riding mower or lawn tractor. While not foolproof, this feature greatly reduces the chances of serious injuries due to accidental slipping or falling - or attempting to make adjustments while the blade is spinning.

    WARNING: Never defeat the dead-man control for any reason unless there is no chance of the mower starting.

    Always restore the dead-man control to normal operation before attempting to start the mower.

    Types of rotary mowers

    In addition to options with respect to gasoline or electric power, rotary mowers can be classified based on such things as: walk-behind vs. ride-on, to bag or not to bag, and rear or side discharge or mulcher:

    WARNING: Most lawn mowing accidents result from reckless or careless use of riding mowers and lawn tractors, not walk-behind mowers. These are not as stable as an automobile especially when their large bags are full of heavy, wet, grass clippings and may tip over on inclines that would not be a problem with someone in control of a walk-behind lawn mower. They are best and safest for use on large flat open lots. A walk-behind mower - perhaps even a manual reel mower - should be used for sloping or irregular areas and for mowing or trimming around obstructions like trees, shrubbery, landscaping timbers, posts, fences, planters, boulders, and so forth.

    Putting a mower into service for the first time

    If this is a new purchase, you should have a users manual - READ IT!!!

    Above all, understand the very important SAFETY information.

    If there is some 'simple assembly required', take your time and follow the instructions step-by-step. Despite the apparent efforts of the designers of the mower and the manual writers to make everything as obscure as possible, it will probably go together without undo difficulty if you use the proper tools. With some, all you need to do is unfold the handle taking care not to pinch any control cables - oops - and you are ready to go to work. Happy times are here again!!

    IMPORTANT: For 4 stroke engines, make sure there is oil in the engine!!! Learn how to check it and fill it to the proper level if there is none or it is low. See the section: Checking the oil.

    WARNING: Running an engine without oil can ruin it in a few minutes and your warranty will not likely cover such stupidity. Since the lawn mower may be shipped without any oil, it is your responsibility to check this and then add the proper amount of the correct type of oil to the crankcase!!!

    For 2 stroke engines, special oil (not the usual 10W-30/40 type motor oil, nor WD40 or 3-In-One!) must be mixed with the gasoline in the correct proportions IN THE GAS CAN - not the fuel tank of the mower! See your users manual! Forgetting to use the proper mixture can ruin a 2 stroke engine in a matter of minutes and your warranty will not likely cover such stupidity. Adding the oil to the fuel tank is not recommended because thorough mixing cannot be assured.

    If you have yard equipment with both 2 stroke and 4 stroke engines, clearly label the two gas cans to indicate which equipment each is used in.


    Use only fresh unleaded gas. Regular octane rating (87) is fine - small engines operate at relatively low compression ratios so knocking should not be a problem. There is likely no benefit to using anything higher. In fact, I've heard that small engines may actually be harder to start and run poorly on expensive high octane gas due to their low compression ratio. Gas that is over a month old may have lost some of its more volatile fractions resulting in hard starting, possible varnish formation, and other undesirable effects. At the end of the season, dump the unused unmixed gas into your car (preferably when its fuel tank is at least half full so your old gas gets mixed with fresh gas). This way you won't be tempted to use it at the start of the next season.

    Note: Apparently using a 2-stroke gasoline mixture in your car may actually cure some types of valve problems, at least temporarily. This is similar to adding stuff like "Marvel Mystery Oil(tm)", but is probably a lot cheaper. You didn't hear it from me though. :) In general, adding some 2-stroke mixture to an automobile's fuel tank isn't going to cause any harm unless it's a high concentration in a steady diet.

    If the tank detaches easily as with many Tecumseh/Craftsman engines, just lift it off and drain the gas into a gasoline safety can reserved for this purpose. If the tank doesn't come off easily, I typically use a cooking baster to do this (you know, the thing you use when baking turkeys!) although the gas tends to destroy the rubber. There are similar devices or gas siphons available at auto parts stores that survive better.

    For 4 stroke engines, gasoline is used as-is since there is a separate oil supply. For 2 stroke engines, you must mix the proper amount of the correct 2 stroke engine oil (outboard motor oil or whatever is recommended by your engine's manufacturer). Fill your '2 stroke mixture' gas can with about half the amount of gas you are preparing and add the proper amount of 2 stroke engine oil. Put on the top and slosh this around to thoroughly mix the oil in with the gas. Then add the remaining gas to the total amount for which your oil measurement was made.

    How do you tell if a lawn mower you just inherited has a 2 stroke or 4 stroke engine? The vast majority are 4 stroke - look for an 'oil filler cap'. On many, this is clearly marked with words like 'oil' or 'oil fill' or with a suitably ambiguous icon. Removing it will reveal a dipstick. (Note that unlike the engine in your automobile, this is both the test and filler location.) However, on more basic models, it may be near the base of the engine and be unmarked. In addition, there will generally be markings as to the need for the gas/oil mixture somewhere on the cover. The only major manufacturer of lawn mowers I know of that has used 2 stroke engines in recent designs extensively is Lawnboy.

    If you are unsure of the correct mixture ratio - they typically range from 16:1 to 32:1 gasoline:oil by volume - 16:1 is probably a safe choice, though newer engines will probably be safe with 32:1 or even higher. The worst that can happen with too low a ratio is that the spark plug may be more likely to foul (and you will pollute more than necessary) but at least you won't risk damaging the internal parts from lack of oil. It is of course best to determine and use the recommended mixture ratio.

    (From: Mike's Small Engine (

    Today's manufactures are having to comply with the EPA standards for small engine emissions. Although you will see gas:oil ratios as high as 40:1 or 50:1, it is perfectly safe to use a little more oil in your gas. For each 1 gallon of gas, I would recommend 4 ounces of 2-stroke oil. This translates to a 32:1 ratio. Since oil is added to the gas mixture to lubricate and protect the internal engine parts, having a little bit more oil will only help your 2 cycle engine last longer.

    Where to buy gasoline

    Your nearest auto service station will sell you small quantities of gasoline. As noted in the section: SAFETY, you must use an approved gasoline safety can which will be red in color. These come in sizes from 1 to 5 gallons or more and may be made of plastic or metal. A convenient size is 2 to 2-1/2 gallons. The fuel tank on a typical walk-behind mower will be between 1/4 and 1/2 gallon.

    WARNING: When filling your gas can, place it on the ground a few feet from your vehicle - never fill it inside the trunk or truck-bed. See the section: Where to fill gas can.

    However, a gallon of gas goes a long way with a walk-behind lawn mower. I don't have exact numbers but a gallon will probably do several acres of mowing (assuming you aren't chopping foot high grass!).

    If you must transport gasoline in a vehicle, make sure the container is secured in an upright position and tightly capped (both the filler spout and vent openings). Provide adequate ventilation so there can be no possibility of fume buildup in the trunk or passenger compartment.

    Where to fill gas can

    Always remove the gas can from vehicle before filling it. In addition to common sense 'what ifs' with respect to spills and fume buildup, there may be an added risk of the buildup of static electricity:

    Also see the section: Explosion risk when filling a metal gas can.

    (From: Joyce (

    Please be careful about filling the gas can. I saw something on one of those PBS How to do it shows (or was it in the newspaper?) that said that people are filling their plastic gas cans with them still in their trunks or hatchbacks. They said this is dangerous because the can isn't grounded or something and a static electricity spark could make the thing to explode.

    Gasoline additives or stabilizers?

    The use of a proper stabilizer can often substitute for the task of draining the gas between seasons by preventing the formation of insoluble gum that would otherwise eventually clog up your carburetor.

    "Isn't there a gasoline additive you can add to the gas which will keep it 'fresh' for up to 24 months? I've seen it at Home Depot, though have never used it. I think a tube was less than $1.00."

    (From: Floyd Reed (

    Being an old small engine mechanic from way back, I suggest that you don't use any additives to keep your gasoline 'fresh'. Dispose of old gasoline (end of season) by dumping it in the (nearly full) fuel tank of your car, then refill your gas can at the beginning of the mowing season. It should last you the season. With this method, you spend no extra on additives (that are no good for an engine), you don't waste 'old' gasoline, and you don't dump raw gasoline into the environment.

    (From: Dan Weise (

    I have found the commercial product Stabil the best thing since sliced bread. It prevents the gasoline in the gas tank from gumming up. I'm too busy to run around emptying gas tanks in the fall.

    Instead, I shoot in a squirt of Stabil, run the engine for 5 minutes, shut the gas valve off (unless it's a suction carb mounted on the gas tank) and the engines start next spring.

    That was *not* the case before Stabil. I can't count the number of times I've disassembled a carb and sprayed Gumout trying to unclog the main jet or the idle jet. Or had the float stick down and flood everything in sight. All because of gummy gasoline.

    I've used Stabil for about 10 years with great success.

    I also use that Slick 50 teflon stuff, but have no idea whether it does any good or not. But for the investment in an engine, I can't see that it hurts. The intermittent duty of many engines makes them more vulnerable to oil run-off than vehicles that are used daily.

    Before you start mowing

    Note: while written with gasoline powered rotary mowers in mind, most of these comments apply to electric models as well.

    Starting the mower

    Most mowers and other small pieces of gas powered yard equipment use a self retracting recoil type starter. You pull on a handle attached to a cord wound around a one-way clutch affair. Pulling on it rotates the engine's crankshaft and the clutch allows the engine to run without pulling you back into the mower! If yours has an electric starter, then you don't need to tug on anything - plug it in and push a button or turn a key. Of course, finding an outlet at the far end of a large yard may prove to be a challenge. With larger equipment like riding mowers, power is usually provided by an on-board rechargeable battery. In either case, there will be some kind of backup recoil or rope starter should the electric start be unusable or inconvenient.

    For the following, we will assume you pull a self-retracting starter rope. With an electric starter, replace the words: 'pull...times' with 'crank for several seconds'.

    Move the mower or other equipment to the place where it will be used - no sense in dragging a chugging lawn mower through the neighborhood. Position it on a solid level surface. Make sure there are no loose stones, twigs, branches, logs, etc. underneath to get sucked up and thrown about once you succeed in getting the engine started (if you ever do).

    When attempting to pull on the starter cord, it may be helpful to put one foot on the mower deck to brace it. Whether this is needed will depend on the design of your mower and in what direction the cord exits from the starter.

    You or the starter motor supplies the power to get it started. However, at the low speed of starting, special modifications may be required to the fuel system for the engine to catch. These may take one of the following forms:

    If the appropriate procedure is not successful, the engine may be flooded. You can give it 15 minutes or so for the gas to evaporate and try again or, if there is an IDLE or LOW speed position, open any choke and pull the rope several times in this position which should clear out the excess gas. Then repeat the recommended starting procedure.

    If none of this works, you may have a starting problem and should refer to the section: Lawn mower will not start. Probably, you forgot to fill the gas tank!

    A primer on priming

    Many small engines have replaced the choke with a primer - a rubber bulb or button that is supposed to be pushed several times before attempting to start the engine. Under the right conditions, this is a very effective approach. However, here are a couple of things to keep in mind:

    Where behavior seems to have changed, first confirm that environmental conditions are the same and the gasoline is fresh before blaming the engine on starting problems.

    If the engine operates normally once started (assuming you can get it started by some other means like squirting some starting fluid into the cylinder), then dirt may have made its way into the priming mechanism. Disassembly and cleaning may be all that is needed. However, there really isn't much to it: Pressing the primer just pushes some air into the carburetor, which squirts some gas via the main carburetor jet to the intake pipe. There really isn't much that can wrong as long as the rubber primer bulb and connecting tubing (if the primer isn't on the carburetor itself) is in good condition.

    Stopping the engine

    On most inexpensive lawn mowers manufactured within the last 10 or 15 years, releasing a dead-man bar on the handle both kills the engine and applies a brake to the blade (well, actually the flywheel of the engine). In this case, there is nothing to think about - just release the handle and it will stop within a second or two. These are quite reliable. The usual problem is that you forget to engage the dead-man bar and attempt to start the mower despite the basic fact that this is quite impossible!

    More expensive equipment will have a blade brake clutch meaning that while there is still a dead-man bar but instead of killing the engine when released, it disengages the blade (clutch) and brings it to a rapid stop (brake). This is more convenient especially with a balky engine. There will then be a separate engine stop switch - possibly combined with a speed/throttle control.

    Equipment with an electric starter may have an ignition switch just like an automobile and there will be three positions: STOP, RUN, START.

    Some older equipment just has a stop contact that grounds the spark plug. Pressing on a lever connects the spark plug terminal to the engine chassis and kills the spark. While this is fairly reliable, it may be a momentary contact meaning that the engine may be on a hair trigger and even rotating the blade a fraction of a turn may cause the engine to take off again. Thus, disconnecting the spark plug wire or removing the spark plug is even more critical when working on this sort of equipment.

    Suppose it fails to stop?

    What should you do if the engine stop switch has no effect? Probably the safest and surest is to use a wooden stick to pop the spark plug connector off of the spark plug terminal. This **will** stop the engine.

    The most likely cause of such misbehavior is a stop wire that has become disconnected or has broken. This is easily remedied.

  • Back to Small Engine Repair FAQ Table of Contents.

    Maintenance Guide

    General preventive maintenance

    Here is what you should do at the end of the mowing season:

    Then, when it comes time to start mowing again (yes, I know, you can't wait), all you should have to do is add fresh gas (don't use last season's). The mower should start on the first (well, maybe, second) pull. There may be some white/blue smoke for a few seconds from it burning off the oil coating on the cylinder walls but this should quickly disappear.

    The Web site has great deal of useful information including diagrams of popular carburetors and links to small engine manufacturers.

    Here is another recommendation:

    (From: Bill Harnell (

    Change the oil at the *end* of the season. No need to leave the acid charged oil in the crankcase over the winter to corrode the engine.

    Then add a couple of teaspoons or so of Stabilit to the gas tank. Run the engine for approximately 5 minutes and while you're at it, inject some fogging oil through the carburetor to thoroughly coat all of the interior surfaces. Directions are provided on the fogging oil container.

    Wipe the frame and handle with an oily cloth and oil all pivot points lightly. Clean the crud from under the deck - you do that frequently all summer - right? Remove all the grass clippings from around the flywheel and the cylinder fins.

    Then store it in the shed or garage.

    It will start on the first or second pull every spring.

    BTW, you should be able to get both Stabilit and fogging oil at any reputable engine service center.

    Checking the oil

    The following applies only to 4 stroke engines. For 2 stroke engines, special oil must be mixed in proper proportion with the gasoline.

    The proper amount of oil is critical to the happiness of your engine. Too little and it may overheat, cause excessive wear, and in extreme cases (but not unusual), cause engine parts to seize and fail - very expensive. Make it a habit to check the oil regularly. Doing this after about every 5 hours of operation is generally recommended. More frequent checks - such as before each time you mow - are fine as well. A typical small engine in reasonably good condition does not use up a lot of oil but checking oil is easy and will not hurt.

    Oil should be checked when the engine is cold or after waiting 10 minutes for it to drain back into the oil sump after running the engine.

    Place the mower on a level area.

  • If there is a dipstick - remove it, wipe it with a rag, and then reinsert it as directed on the information sticker or on the stick itself - usually threaded fully back in place and then removed to read. Look at both sides of the dipstick to make sure it didn't just pick up oil from the filler tube AND reoeat the entire procedure be sure of the results. If the level is below or near the empty mark, slowly add the proper new oil to bring it up to just below the full mark.

  • If there is just a filler plug near the base of the engine, the correct level is just below the top - almost to overflowing. If lower than 1/4" or so below the lip, add new oil to top it off. (There may be exceptions to this but filling to near the top should be safe if you do not have your lawn mower engine manual handy.)

    If the oil level is high - you just bought the lawn mower or were careless in filling it last time - drain enough oil to bring the level back down to the full mark. Too much oil can result in problems as well - oil spraying out of various orifices or getting into other places where it should not be like the combustion chamber.

    If you find the oil level over the full mark or higher than it was before, gas may be leaking into the oil due to a flooded carburetor - a stuck inlet needle or bad float. If this is the case, the oil will need to be changed once the underlying cause of the leakage is determined. (This is only likely with float type carburetors such as those used on the Tecumseh engines used in a variety of Sears/Craftsman models equipment.

    If the oil is very low and you have been performing regular maintenance, there may be a leak or your engine may need a ring job. Excessive oil under the deck - on the shaft or blade adapter - would indicate a bad bearing or oil seal. Noticeable blue smoke while running would indicate that excessive oil is getting by the rings into the combustion chamber.

    Typical oil capacity is just over 1/2 quart (usually about 1-1/4 pints).

    About engine oil

    For the common 4 stroke engines such as Briggs & Stratton or Tecumseh, you don't need to buy specially high priced engine oil. The type likely used in your automobile will work fine and will be much less expensive. Although small engine manufacturers may recommend SAE 30 oil, they usually also state that 10W-30 or 10W-40 are acceptable substitutes - and these are what your car probably uses. New oil should have at least the SG rating though if you have some SF in your garage, that will be fine also (for your lawn mower, not your car!). However, there are situations like equipment that must run in extremely cold weather (probably not a problem with grass mowing!) where specific alternative oil recommendations should be followed.

    The capacity of a typical small engine is just over a 1/2 quart. This will probably cost you about 50 cents - a very worthwhile investment!

    However, some people do swear by synthetic oil. I remember it worked pretty well on my bicycle as well :-):

    (From: Daniel Pope (

    Synthetic Motor Oil in the crankcase and Marvel oil in the gas is the only way to go!

    I have a $1,900 MTD tractor style mower with B&S engine (L head twin). It has over 800 hours and 9 years on it and I mean rough hours (bahaya grass and hot weather). The engine does not use a drop of oil (changed every 30 hours) and the compression is the same as when new. These other guys can have there $5,000 fancy mowers. I'll use that money to buy a truck.

    Oil change

    If you don't do anything else to prolong the life and happiness of your small engines (and your automobile, for that matter) it should be an oil change at the recommended interval. Oil loses its lubrication and cooling effectiveness with use and this will gradually take its toll on your precision engine parts. Even a simple lawn mower engine is machined to very precise tolerances and any contamination will increase wear. Ultimately, its performance - starting and running - will suffer and its life will be shortened.

    Since there is typically no oil filter, all the grit, metal particles, and other undesirable stuff continues to circulate with the oil to find its way in between precision engine parts.

    For small engines, the oil change interval is usually specified to be about 25 hours of use. More frequent oil changes may be desirable if the equipment is operated in an extremely dusty environment.

    A typical mowing season for a modest size lot is around this amount of time so an oil change once a season is probably satisfactory. I recommend this be done at the end of the season so that the old contaminated oil does not sit in the crankcase during the winter months and you will not (conveniently) forget to do this at the beginning of next season when you are eager to get at that straggly lawn.

    An oil change isn't really a big deal

    The oil change procedure is a lot simpler than for an automobile as everything is readily accessible and there is no oil filter to worry about.

    Blade sharpening

    The blade in a rotary lawn mower doesn't need to be sharp as a carving knife or scalpel but serious dents and nicks will result in a less than perfect lawn (to say the least!). In addition, a seriously unbalanced blade can result in excessive vibration and eventually, possible internal damage as well.

    First, drain the gas or remove the gas tank. If you will be filing steel, you get sparks. Sparks are not the greatest thing to have around gasoline vapor. Enough said. Disconnect the spark plug wire and tie it safely away from the spark plug or remove the spark plug entirely. Turn the mower on its side.

    CAUTION: Immediately check for oil leaks at the oil filler pipe or elsewhere. If there are any, you will need to work on the mower just propped up by 45 degrees or so. Or, use this as a good excuse to perform an oil change and drain the oil (even if the engine is cold, most of the oil will drain out but it will take a little longer). Just don't forget to refill the crankcase with fresh oil once you have completed work on the blade!

    Check that the blade isn't bent. Locate a reference point on one side and note the height of the blade tip at that location. Rotate the blade 180 degrees and check the height of the opposite blade tip. There should be no significant difference - say no more than 1/8" or so. If it is greater, the blade is bent or the crankshaft is bent. Either will require further investigation as running the mower under such conditions will probably result in excessive vibration and can be dangerous.

    Assuming this is fine, inspect the blade:

    Slight nicks and dents can be cleaned up with a file while the blade is still installed on the mower. Unless you have run into a curb, this is probably all that is needed on an occasional basis. Removing this small amount of metal will also not unbalance the blade enough to worry about. Refer to the section: Non-violent blade removal if it needs to come off the mower.

    If the damage is severe, consider replacing the blade entirely - they are not that expensive (usually under $10). Otherwise, you can use a file, a bench grinding wheel, or a grinding wheel mounted in an electric drill (there are special attachments for this specific application).

    Since the rotating blade also contributes to the proper air flow, you do not want to upset the shape. Grind in such a way that the original blade angle is preserved. It doesn't need (or want) to be razor sharp. A 1/64" edge is fine. Anything finer will quickly be dulled by little bits of stone and dirt in any case. Safety is not the main concern here - if any part of your anatomy contacts the whirling blade, you **will** be in trouble no matter how dull or sharp the blade might be!

    Attempt to remove approximately equal amounts of metal from both ends and in roughly similar areas if possible. If there are a few large nicks, it isn't necessary to remove them completely - your lawn (and neighbors) will never know the difference.

    Check the balance by positioning the blade at the center hole location on a pencil or other rod - you don't need a fancy blade balancer but can use one if you like. If it tips one way or the other, remove more material from the heavy side a little at a time.

    Replace the blade along with all its mounting hardware. Make sure you get all parts in the same relationship as they had originally. The blade must have its sharpened edges pointing downward. Don't forget the install the key if it is separate and DO NOT substitute a hard steel key for the soft metal one that should be used. See the section: Why soft metal keys must be used. If the locking key or blade adapter key appear damaged in any way, replace it.

    Non-violent blade removal

    For just some minor touch up, there is no real need to remove the blade. For major grinding and balancing, removal will be needed. Removal will also be required to inspect for a damaged or sheared blade lock key and to replace it if necessary.

    In either case: disconnect the spark plug wire and tie it safely away from the spark plug terminal (several inches minimum) or remove the spark plug entirely to prevent accidental starting.

    If the nut holding the blade on is just on very tight, use a block of wood to prevent the blade from turning. Use a good quality socket wrench or box-end wrench of the correct size - an adjustable or even open-end wrench may not be enough. The nut usually unscrews counter-clockwise. However, check this out first! A careful inspection of the threads on the end of the crankshaft will reveal the direction. Or, determine the direction of rotation which will be designed to tighten, not loosen the blade. Most, if not all, single blade mowers rotate the blades clockwise as viewed from above which will therefore use a normal right-hand thread nut.

    CAUTION: Make sure that if the wrench slips, your flesh will not contact the blade or other sharp sheet metal - liberal use of rags or newspapers is a good idea. Arrange your position and the mower so you are pulling towards you - this is a more stable controllable arrangement.

    (From: Graduate student of school of hard knocks.)

    "I'd wish I'd read this a few years ago. I used an open-end wrench and it 'flexed' off of the bolt. Needless to say, my next week wasn't a lot of fun with 10 stitches in my hand."

    Use some penetrating oil (e.g., liquid wrench or WD40) on the nut and threads if there are signs of rust or corrosion. Allow it to soak in for a few minutes before attempting to remove the nut.

    You will prevail. A hammer or other more violent approaches should not be needed.

    Once the nut is loose, unscrew it the rest of the way by hand and remove any washers or mounting plate and note their exact position and orientation. The blade and adapter should come off easily. Some penetrating oil (e.g., WD40) may help if it is difficult to remove.

    If your adapter/blade doesn't pop off after removing the nut or bolt, it may be mounted using a taper like the flywheel. This is somewhat unusual on a walk-behind lawn mower but might be present on a larger machine like a lawn tractor. A wheel puller is best for dealing with this situation but first see if it isn't just gummed up or rusted in position - try the WD40.

    Inspect the key or locking tab for damage. You may have:

    If the adapter's tab is broken off or the key is sheared or damaged, then replacement of the entire blade adapter or just the key (depending on your mower's design) will be needed upon reassembly. For now, if you will be sharpening the blade, replace all the hardware in the correct positions (except the blade) and finger tighten the nut so you won't lose anything.

    WARNING: Do not install a hard steel key in place of the recommended blade lock key as you will lose the protection that the soft metal provides and the next incident may be the last... See the section:Why soft metal keys must be used.

    Once you have reground the blade or obtained a replacement, reassemble in reverse order and then tighten the nut to the proper torque.

    (From: Gib Gahan (

    Another way to remove a stubborn blade is to take it to your friendly garage or tire changer and have them put an impact wrench on it. Saves knuckles, tempers, etc. Just don't put the blade back on without a touch of oil or anti-seize compound and of course, don't use an impact wrench!

    Carburetor adjustments

    If your engine is relatively new (made within the last 10 years or so) and has only one speed, then there may be no adjustments (like the one discussed in the section: Cleaning Craftsman (Tecumseh) carburetors. It will either work or it won't - in which case it needs cleaning or parts replaced. Or your overhaul was less than 100% effective. You can tell if your carburetor is of this type as there will be no adjusting screws on the carburetor. For Craftsman types, there will be a solid hex nut on the bottom holding the float bowl in place. There may or may not be a primer button.

    For adjustable types, it is best to refer to your engine manual. However, here is the general procedure. Some of the specific numbers may differ for your engine, however.

    In all cases, before touching any adjustments, make sure your air filter is in place, and clean (or new). Fill the fuel tank about half full with fresh gasoline.

    There are three adjustments on a typical carburetor:

    Initially, carefully and gently turn the two mixture controls in until they just seat.

    Note: "In" means clockwise (the way you would tighten a normal screw) and "out" means counterclockwise (the way you would loosen a normal screw).

    CAUTION: do not force them - you are not trying to tighten anything - as you will damage the needles and seats which will require replacement of the needles or entire carburetor. Then back them out 1 to 1-1/2 turns. Set the idle speed screw 1 to 2 turns beyond where it contacts the throttle plate. Refer to your engine manual for specific recommendations! These settings should allow the engine to start and run, though perhaps not entirely smoothly or with great enthusiasm.

    Notes on Briggs & Stratton tune-up

    (This assumes foam type air filter that needs have a film of engine oil in it to trap dust.)

    (From: (

    Once the air filter is saturated it must be wrung out. I typically place them in a paper towel and squeeze. Then possibly a second paper towel. No oil should be dripping out of the filter when gently squeezed. Chilten's B&S engine section says to simply squeeze the filter of excess oil - no mention of a paper towel, etc.

    In adjusting the carburetor, Chilten says about 1-1/2 turns but 2 turns is my standard.

    I get the engine running first - warm it up to general operating temp. No applied choke, air cleaner installed. Adjust the main jet if you can get it to run at rated speed - preferably under load (turn in to the lean studder, out to the rich studder then back in about 1/2 way between these extremes). Then adjust the idle, at idle - at no load - using the same operation as the main jet. You may then go back and repeat/refine the process a second time as the idle adjustment may affect the main a bit.

    For the 92000 in particular (typical but refer to your specific engine model for exact specifications), from Chilten's second edition, "Repair & Tune-Up Guide for Small Engines" (successor to [2]):

    Setting engine speed

    Many inexpensive mowers don't even have a speed adjustment screw. Spring tension and the linkage to the governor set speed - period. Note that actual speed is rarely that critical for engine driven equipment as long as it is within safe limits. See the comments below on string trimmer speed settings. However, what options do you have where there is a need to set the precise engine speed?

    For equipment where a shaft with a known speed ratio to the engine crankshaft is available (i.e., a power take-off or trimmer head), an optical stroboscope of one form or another may be used. You will just need to paint or tape some stripes on the rotating part to put under strobe illumination:

    For example, for something like a string trimmer which has a direct coupled hub, strobe disks with 2, and 3, and 4 radial lines will appear stationary under fluorescent illumination for 3,600, 2,400, and 1,800 RPM respectively.

    There can be ambiguity but if you are already in the ball park, this sort of approach may be all you need.

    (From: Philippe Habib (

    Go to a hobby shop that sells radio controlled airplanes. Plunk down $30 or so and get an optical tach. Paint 2 strips on the (crankshaft) hub of your equipment to simulate a propeller, and you're done.

    (From: J. Matthew Good (

    You shouldn't need a tachometer on a trimmer. Two stroke engines in the size and power range of line trimmers can't overspeed as they don't have the power with a line head installed. Just set the carburetor up so that it 'two-fours' at wide open throttle and you should be all set. If it needed to be adjusted with a tachometer, it would have some kind of governor on it.

    (From: Mowerman (

    B&S engines have a spring in the governor arrangement. You want to change tension on the spring to change the speed. (This is basically true of most other small engines as well but the details will differ.) The spring is attached to a metal tung (strip) in the linkage at front of motor, this is made of a tinny metal so you can easy bend this tung. To lower speed you want to lessen the tension on the spring. You can do this while the motor is running at top speed but it would be safer to do the bending while mower is not running. By pushing in tung you will lessen top speed gently as it doesn't take much to alter that speed. Often this tung gets knocked in while mowing around bushes or other protruding material and "hey presto" your engine is only idling. It is a design problem that briggs should be working on, however I love B&S engines with their simplicity and ease of maintenance.

    Adjusting the carburetor on a 2 stroke engine

    This is generic advice but probably a good place to start. It assumes that there are 2 adjusting screws - idle and high speed mixture.


    I would start with them both backed out from the closed position to 2 turns open, for starters. One should be idle mix and the other should be high speed. Crank the motor, get it warmed up. You may have to fiddle with the throttle and or choke until its warmed up. Then slowly turn in the low speed jet, until it starts to die, then back it out another 1/4 to 1/2 turn. Then hold the throttle wide open, and slowly turn in the high speed, until it really starts to smooth out and rev high, and start screaming, then back this out until it starts to run rough or slow down, and then turn it back in to midway between these two positions. Keep playing with the low speed needle until you get rapid immediate response from the throttle, and good idle with the idle adjustment. Then play with the high speed needle, but always back it out from the setting about 1/4 turn or so, from where it runs the smoothest. This setting of backing it out will allow more fuel when under a load, and keep your 2 cycle motor from running too lean. Put a load on it and just tweak the settings just a hair at a time. If you're in the ball park it won't take much adjustment either way to make a difference. Better on the rich side than lean side. Also make sure your oil / gas is mixed at the proper ratio. 99% of 2 cycle motors are adjusted the same, no mater what brand they are. Most will start and run with both screws open 2 turns initially. Just don't close the high speed off any more than necessary no matter how well it runs.

  • Back to Small Engine Repair FAQ Table of Contents.

    Troubleshooting Guide

    Instant troubleshooting chart - most common problems and possible causes

    The following chart lists a variety of common problems and nearly all possible causes. Diagnostic procedures will then be needed to determine which actually apply. The 'possible causes' are listed in *approximate* order of likelihood. Most of these problems are covered in more detail elsewhere in this document.

    While this chart lists many problems, it is does not cover everything that can go wrong. However, it can be a starting point for guiding your thinking in the proper direction. Even if not listed here, your particular problem may still be dealt with elsewhere in this document.

    (Portions of the following from: Chilten, Small Engine Repair 2-12 HP, (1).)

    Lawn mower will not start

    This is probably the most common problem you are likely to encounter. The cause is very often the same - lack of maintenance.

    Note that the assumption here is that it cranks - the crankshaft and blade rotates in a normal manner but the engine never catches. Some larger (Briggs and Stratton) engines may have a low-oil cutoff switch which will stop the engine if the oil level is inadequate. However, this is not likely on a push mower.

    If you are unable to pull the cord (or the auxiliary starter on one with electric start), there may be a clump of grass stuck between the blade and the deck or there could be serious internal damage, especially if you just encountered an immovable object. See the section: Lawn mower will not start after the blade hit an obstruction. However, you didn't forget to engage the dead-man bar, did you? On most inexpensive mowers this safety interlock is needed to both enable the ignition system and release the blade brake.

    Lawn mower is hard to start

    Most of the time, the possible causes and solutions will be similar to those where the engine doesn't start at all. So, see the following sections for more information. However, here are some specific issues dealing with engines that do start eventually and then run fine:

    (From: Mike's Small Engine (

    Nowadays, a lot of mower/equipment is using OHV (OverHead Valve) style engines. This in itself causes many starting troubles. These engine use a compression release on the camshaft to allow for easy starting. Once the engine has started and reaches a speed of over 700 rpm, the weight on the compression release swings out and allows the engine to gain full compression and full power. After awhile, the valve lash/clearance begins to widen and the first thing that goes wrong is the compression release doesn't work properly causing the engine to drag which may sound to some as a dead or dying battery. I'm sure there are untold thousands of people that have replaced batteries, solenoids, starters, switches, etc., only to find themselves going to a repair shop and letting a trained mechanic do what is a simple maintenance adjustment.

    Determining why it won't start

    Think: FAST - Fuel, Air, Spark, Timing. Diagnosing a balky engine is not difficult but a step-by-step methodical procedure will make it a lot less traumatic. Despite all the warnings, serious problems rarely develop on their own. Most likely, there is a simple, easily remedied cause.


    Obviously, the engine won't run without gas!

    Once you have exhausted these obvious problems, determine if gas is reaching the cylinder as follows: Perform the normal starting sequence and then, assuming it shows no signs of wanting to start, immediately remove the spark plug. If fuel is reaching the cylinder, the spark plug should be damp with gas and there should be a very distinct odor of gas from the spark plug hole. If there is none, then there could still be a blockage in the fuel line or the carburetor may need cleaning.

    A flooded engine, most likely due to extended unsuccessful attempts at starting or a defective carburetor (float valve stuck open or gas-logged float) will result in inability to start as well and a distinct odor of gas. You might find raw gas coming our of various orifices - air filter as well as exhaust. (Note that in severe cases, enough gas gets mixed in with the oil to significantly increase the level in the crankcase and reduce the effectiveness of the oil. This will require an oil change.


    The optimal air:fuel ratio is around 14:1. This must be lower for a cold engine and thus a choke plate or other means to increase the richness of the mixture is usually provided. A choke plate restricts air intake forcing more gas to be sucked into the cylinder. A primer bulb effectively squirts gas into the intake pipe to augment the normal carburetor action. Some carburetors have no choke and no primer but incorporate a small gas reservoir which fills when the engine is off and provides some extra when starting.

    To much air results in a mixture that is too lean, burns too quickly, and can result in engine damage over extended periods of operation.

    Too little air results in a mixture that is too rich - there will be loss of power and possibly black smoke from the exhaust. This could be due to several factors:


    All common lawn mower engines require a precisely timed spark to ignite the air-fuel mixture in the cylinder. The existence of a spark can easily be tested as follows:

    WARNING: make sure there is no gas in the vicinity when performing the following test!

    Remove the spark plug wire and insert the blade tip of an appropriately sized and well insulated (plastic) screwdriver inside the boot or clip in place of the spark plug. While holding the *insulated* part of the screwdriver, position the metal part of the blade about 1/8th inch from the block or frame.

    An alternative technique is to use an old, but good, spark plug whose gap has been increased to about 1/8 inch or one specially made for exactly this purpose. In this case, simply connect the spark plug wire to the test plug and hold its threaded part against the cylinder head or other part of the chassis (away from the gas tank!!).

    Note: Just positioning the spark plug wire a short distance from the spark plug terminal is not recommended as the results of this test will then depend on the condition of the spark plug as well since the spark will have to jump two gaps.

    Have a buddy crank the engine at normal starting speed so that you will be able to hold the screwdriver or test plug steady and be close enough to see any spark clearly. Shield the gap from the sun or bright light if necessary.

    You should see a nice healthy spark jump the gap several times on each pull (actually, once per rotation of the crankshaft/blade on both 2 and 4 stroke engines). Note: 4 stroke engines ignite the air-fuel mixture on every other rotation of the crankshaft. The extra sparks fire harmlessly into the exhaust gasses and are wasted. Can you believe it?!

    CAUTION: if you are not well enough insulated, *you* will jump several times per rotation of the crankshaft/blade if the ignition system is functioning properly! Hey, that *is* a valid test!

    If this test confirms the spark, it is still possible that the spark plug is fouled or bad. See the section: Checking the spark plug.

    If there is no spark, then there is a problem with your ignition system.

    Lack of spark

    If your mower is less than 15 years old, there is an excellent chance that if uses an electronic ignition system. These are very reliable as there are no points or condenser to go bad and no need for routine tune-ups.

    However, a number of other problems can result in lack of spark:

    Make sure stop switch/stop wire is in appropriate position - confirm with a multimeter, check that flywheel is being spun by starter and that flywheel key is intact to assure proper timing, check condition of points/condenser and setting (if applicable), test magnet (on flywheel) for strength, check the gap between flywheel and magneto core. If these are all fine, test or replace the magneto.

    In more detail:

    1. Check for a faulty or misadjusted STOP switch. This may be activated by releasing the dead-man bar or by a throttle control lever (STOP, RUN, START). Inspect the cable, linkage, and wiring for damage or for something that may have come loose. Make sure you have the controls set properly to run!

    2. Check that your starter is actually spinning the flywheel. If the flywheel is not rotating properly when you pull the cord or turn the electric start key, then there is a problem with the starter, not the ignition system. Or, the flywheel is not tight due to a sheared flywheel key or improperly torqued flywheel nut.

    3. Check for a flywheel that is loose and not seating properly on the taper. This could result in no spark if the air gap between the flywheel magnet and magneto core is then incorrect. However, due to the close spacing, you would probably feel and hear serious scraping in this case.

    Items (2) and (3) are likely if your just attempted to move a curb with your mower blade (or if someone inadequately tightened the flywheel nut during some previous maintenance).

    1. Check for bad connections or defective wiring including faulty or water logged insulation. If you just gave the mower a shower, wait ample time for it to dry out. High humidity may result in more problems if the insulation is not in good condition as well.

    2. Check for a weak (or missing) flywheel magnet. Both of these faults are extremely unlikely unless you have been hammering and whacking the crankshaft and flywheel in an effort to remove the flywheel. (This is not recommended - see the section: Flywheel removal.)

    3. (a) Electronic ignition - There is likely a single potted module which includes the circuitry and ignition coil. If anything goes wrong with this module, replacement is the only option. Once the wiring and resistance of the secondary has been checked, there are really no addition tests that can be performed on an electronic ignition module without special equipment. A defective ignition module will have to be replaced.
    1. (b) Breaker point ignition - Possibilities are bad, dirty, corroded, or loose points or points that are grossly out of adjustment, a bad condenser, or a bad magneto coil. See the section: Maintenance of point-type ignition systems.

    See the section: Testing the magneto.

    First, check that the dead-man bar is properly disengaging the stop switch when pulled and/or throttle control is properly disengaging the stop switch when in the start or run position.

    For anything beyond this, disassembly will be needed to identify and replace any defective parts.

    If the no-spark condition happened after the blade hit an obstruction, (1) or (2) are likely. See the section: Lawn mower will not start after the blade hit an obstruction.

    Checking the spark plug

    Use the proper socket to remove the spark plug and inspect it for damage and general appearance:

    The best thing to do at this point is just replace it with a new spark plug and worry about the old one later. Actually, nearly every small engine maintenance book will recommend changing the spark plug every season anyhow.

    Testing the magneto

    The magneto, like the ignition coil on an automobile, contains two windings:

  • A primary with a few turns of heavy wire.

  • A high voltage secondary with thousands of turns of super fine wire.

    In an automobile, the battery supplies the primary current; in a magneto, the magnet on the flywheel moving past the core at high speed acts as a generator and induces current in the primary.

    As the magnets spin past the pole pieces of the magneto core, the points are closed and current builds up in the low voltage winding (and flux builds up in the core). At or slightly before Top Dead Center (TDC), the current (and flux) should be maximum and at this instant the points open. The flux then collapses (and the condenser (capacitor) across the points acts as a snubber allowing the current to bypass the open points and preventing arcing at the point contacts). This rapid decrease in flux results in coupling of the stored energy to the turn high voltage winding and results in up to 10,000 V or more at the spark plug.

    (For EE types, this is somewhat similar in basic operation to the flyback converter in a switchmode power supply except that the moving magnet supplies the input power instead of the rectified AC line and the points act as the switch instead of a power transistor.)

    The secondary will always be accessible for testing but the primary of an electronic ignition may be not be due to the electronic components:

    Wires can break due to corrosion or vibration. This would result in an open winding - infinite resistance. Shorts can develop between adjacent windings or to the core. This may be detectable as reduced resistance but without knowing exactly what it should be, there is no way of knowing if a slight discrepancy represents a problem or just slight variations in design or manufacturing.

    A more complete test would involve checking the 'Q' or doing what is called a 'ring' test and even more for an electronic ignition. This requires special equipment. Therefore, it is best to swap in a known good unit. They are not that expensive.


    For power to be developed, the ignition of the compressed air/fuel mixture must take place at exactly the correct instant - just before the piston reaches Top Dead Center (TDC) on the compression stroke. With automotive engines, there are mechanisms to advance the spark at higher revs but simple lawn mower engines do not have this complication, at least.

    Timing is set on older mowers with point type ignition systems by adjusting the point gap and generally only changes due to wear. However, these changes are gradual and unless the points come loose for some reason, will not likely suddenly prevent the mower from starting. On newer electronic ignition systems, there is basically no adjustment as the position of the electronic ignition coil/module fully determines ignition timing and this is fixed.

    However, timing can be grossly messed up if the flywheel key gets sheared and the flywheel then rotates a fraction of a turn on its mount on the crankshaft. The result may be a mower that does not start, backfires or runs erratically, lacks power, won't run and/or start when hot, etc. This is very likely to happen should the blade strike a rigid object causing the mower to stop instantly. In this case one or both of the blade lock key and flywheel key have sheared to (hopefully) protect the very expensive internal parts from damage.

    There are likely not going to be any timing marks for that old timing light you have sitting gathering dust somewhere. The only test really is to inspect the flywheel keyway to determine if damage has occurred.

    See the section: Lawn mower will not start after the blade hit an obstruction as this is the most likely cause of a sheared flywheel key.

    Lawn mower will not start after the blade hit an obstruction

    The following description applies to most small rotary lawn mowers with direct driven blades. The vast majority of these use either Tecumseh (as found a variety of Sears/Craftsman equipment) or Briggs & Stratton engines. However, similar comments apply to others as well including Lawnboy two stroke engines and the more modern Honda and other overhead valve type of engines.

    The assumption is that the engine started and ran normally prior to the incident. Now, no matter how many times you yank the starter rope or run the electric starter, it will not start at all, bucks, kicks back, backfires, or fails to develop enough power to keep going on its own.

    If the blade struck a solid boulder while the engine was set on 'high', more severe damage is possible as even with soft metal keys locking the blade and flywheel to the crankshaft, the inertia of the rotating blade is acting sideways against the crankshaft in addition to suddenly stopping its rotation. This can result in a bent crankshaft. The end of the crankshaft with the blade adapter could be bent without affecting the bearings or internal parts. This would need to be tested for as well. Not that such an occurrence is that much better - the crankshaft would still have to be replaced but at least the bearings in the crankcase will not be damaged.

    If the starter will not turn the crankshaft (assuming you remembered in your haste to engage the safety bar) - it is seized or will only rotate part of a revolution before hitting against something solid inside - then you probably have serious internal damage that will require a complete strip down and replacement of some (expensive) parts. If it turns but much more tightly than you recall (assuming you do have the safety bar engaged!) then the crankshaft may be bent - again very expensive. Repair may not be worth it.

    However, in most cases, what has happened is that either or both of the blade lock key and/or flywheel key have sheared to protect the crankshaft from serious (and terminal) damage.

    If the blade lock key broke, the blade will no longer turn rigidly with the crankshaft and provide the inertia required by many small engines with undersized flywheels. In this case, the engine may try to start but die out with a few "putt-putts" or even kick back on the starter cord. (As a side note, attempting to use a lawn mower engine as a replacement on a piece of equipment that doesn't have something to substitute for the blade's inertia may not work for this reason.)

    If the flywheel key broke, the ignition timing will likely be totally wrong and the result may be no ignition, backfiring, kickback, or weak or total loss of power.

    To diagnose, proceed as follows:

    First, pull off the spark plug wire and tie it securely away from the spark plug terminal (several inches minimum) or remove the spark plug entirely so that there is no chance of the engine accidentally starting. Even though it will not start now no matter what you do, the underlying problem could actually be a flooded carburetor or something else which may correct itself while you are working. Never take chances.

    Drain the gas or remove the fuel tank. This will prevent gasoline from spilling out the gas cap vent hole or flooding the engine through the carburetor since you will need to tip the mower to get underneath.

    Set the mower on its side (carburetor side up).

    CAUTION: Immediately check for oil leaks at the oil filler pipe or elsewhere.

    The mower can usually be set on its side for a few minutes without harm but if these occur - you will have to work with it tipped less than 45 degrees or so - propped on wood blocks. Or, use this as a good excuse to perform an oil change and drain the oil (even if the engine is cold, most of the oil will drain out - it will just take a little longer). Just don't forget to refill the crankcase with fresh oil once you have completed your work!

    Using an old rag and/or proper work gloves, grasp the blade and attempt to rotate the blade and crankshaft.

    CAUTION, despite your lack of maintenance, the blade may be sharp!).

    The blade and crankshaft should rotate together. If there is slippage, the key has broken and will require replacement of just the key or the entire blade adapter plate depending on design. If it appears to be intact, then you can assume the flywheel key has broken. The blade key may be broken as well but it is not likely the reason for your failure to start. You should remove the blade to determine this for sure before restoring the mower to service in any case. See the section: Non-violent blade removal.

    You can possibly avoid removing the flywheel for inspection of the key by unscrewing the sparkplug, rotating the crankshaft so the piston is at TDC, and noting the location of the magnet on the flywheel relative to the magneto coil pole pieces. The magnet should be pretty close to the magneto in that position. If this is not the case or just to be sure, the flywheel will have to come off to inspect and possibly replace the key.

    To get at the flywheel key itself, some disassembly is required.

    Flywheel removal

    Remove the shroud (blower cover) if you have not done so already. This is usually fastened with 4 screws and hopefully does not involve any head bolts - if so, you will need to tighten them to the proper torque using a torque wrench once you have remedied the problem. You may need to remove the fuel tank (if you have not done this already) and other trim pieces as well.

    You should now see the top of the flywheel. In most cases, a large nut fastens the flywheel to the crankshaft. (However, in some designs, part of the starter mechanism is actually used and this is supposed to require a special wrench to remove. However, using a piece of wood as a buffer and tapping the ears in a counterclockwise direction will work also. Refer to your engine manual for details.) Use the proper socket to loosen this nut (counterclockwise). It may be necessary to brace the flywheel securely to gain enough leverage. Make sure this is done against something that can stand the force. Once loose, remove it by hand and then remove any washers or other parts that are under it. Make a note of how these were positioned including which side is up on some cupped washers.

    You should now see the keyway. The slots on the crankshaft and flywheel should be aligned. There are two common types of keys:

    You may even find that the flywheel is relatively loose on the crankshaft if rotating the blade while holding the flywheel stationary is possible. Either the blade key or the flywheel key or both are broken in this case.

    You will have to remove the flywheel to replace the key if it is broken or damaged.

    If the flywheel is loose at this point, then the following will not be needed as it can be lifted off.

    There are several approaches to flywheel removal:

    Flywheel removal on Briggs & Stratton engines

    The following applies to most B&S engines with the square starter shaft. This procedure will be needed to gain access to the points and condenser (on engines without electronic ignition), oil seal, etc.

    (From: Foxeye (

    The square shaft which turns one direction but not the other is the recoil starter pawl. This can be pulled straight up from the round cup that it sits in. There should be a couple of steel balls inside. Some models have a large internal snap ring that holds this in the cup. Remove ring, and pull it straight up. Use a magnet to remove the steel balls. Once you have this starter pawl removed, and the steel balls, hold the flywheel, stationary, and using a block of wood, bump one of the lugs on the outside of the starter cup in a CCW direction.

    Once you break it free of its torque, it should be able to be spun off CCW. Remove this starter cup, and the beveled bellville washer under it. Now your ready for your flywheel puller. A strap wrench can also be used to turn this starter cup loose. When installing the starter cup, make sure it is clean and free of grease and oils. A drop of oil or a light coating of grease on the crankshafts stub end (over which the starter pawl goes) is usually recommended. The starter cup only needs to be snugged down, and you don't have to go overboard trying to torque it to ungodly tightness. Snug is sufficient.

    Drop in the starter pawl, into the cup over the stub of the crank, and drop in the steel balls, replace larger flat cover over this assy, and insert snap ring if there was one.

    Flywheel inspection

    Once the flywheel is off, inspect the keyway on the crankshaft and flywheel for damage. Serious damage will require replacement of the affected parts. Slight burrs can be removed with a small file. If there are any cracks in the flywheel radiating from the hole, the flywheel MUST be replaced as this is a serious safety risk - the flywheel could literally explode when run at full speed. However, don't be concerned by surface flash - lines that look like fine cracks resulting from the molding process. To confirm that these are not cracks, there will be no visible penetration inside the shaft hole and fine sanding will quickly remove all traces of this flash.

    Assuming there is no serious damage, a new flywheel key should be all you need - about 25 or 30 cents. To confirm that this is all you need, replace the flywheel without the key but line up the two slots as they would be if a key were present. Tighten securely (but it doesn't need to be to the full torque as this is just a test). This should permit the mower to start and run normally but I would not recommend using the mower to actually cut grass until you replace the flywheel key.

    To install the new one, insert the key into the slot in the flywheel first and then slip the entire affair onto the crankshaft (I like to use a bit of WD40 for protection as well). The flywheel should seat securely with no detectable free play - it should be on straight and not rock back and forth at all. If this is not the case, the key may be in upside-down or there may be something or some particles of dirt or metal blocking it. Replace the washers, dirt screen, etc., and then hand thread the flywheel nut as far as it will go. Tighten to the specified torque (typically, 30-33 ft-lbs).

    Note: There may be a cupped washer between the nut/screen and flywheel. This must be installed cupped-side facing the flywheel or else you will be probably be replacing the flywheel key again very soon :-(.

    WARNING: Do not install a hard steel key in place of the recommended flywheel key as you will lose the protection that the soft metal provides and the next incident may be the last... See the section: Why soft metal keys must be used.

    Then, replace the shroud, fuel tank, etc. If head bolts had to be removed, it is probably a good idea to slightly loosen all of the head bolts and then retorque them to the proper value in the recommended sequence for your engine.

    Why soft metal keys must be used

    Normally, the soft metal keys lock the blade and flywheel to the crankshaft. However, should the blade strike an obstacle and stop suddenly, one or both key(s) will shear and reduce the likelihood that the very expensive crankshaft or other parts will be damaged. The reason is that the substantial inertia of the crankshaft and that of the flywheel will tend to try to keep them rotating. Something has to give and you want it to be the 25 cent key and not the $75 crankshaft! However, this isn't foolproof as explained in the section: Why you really don't want to attempt to move an immovable object.

    Note that the soft metal flywheel key can also be damaged without totally shearing which may result in slightly incorrect timing. Symptoms may include a mower that is hard to start, runs rough or lacks power, or cannot be restarted when hot. Therefore, always replace the key if there are any signs of damage or wear.

    Engine dies or won't restart when hot

    (From: Wild Bill (

    There can be numerous reasons for the engine to quit running when it gets up to operating temperature.. expansion of metal parts where bolts/screws aren't tight enough, breakdown of the condenser for the ignition points (if equipped), or a possibility of a crack somewhere. An overall inspection of the tightness of fasteners would be a good place to start.

    (From: Walt Conner (

    As soon as the engine shuts down, turn the flywheel by hand or DISCONNECT the plug wire and turn the blade by hand to see if there is any compression. I have had several B&S engines that did not have enough valve clearance and when hot, the valve stem expands in length enough that the valve does not properly close. After cooling, engine will be OK. Also could be a bad condenser or the other faults listed above.

    (From: Michael Stevenson (

    Problems with the ignition coil can make an engine impossible to start when warm (or even stop running when it warms up), apparently this happens on motorbike engines quite often and is caused by a break in the HT coil.

    A carbon track forms inside the coil where the wire is broken. When the engine is cold the carbon track has a low resistance and it conducts well enough so the spark is strong enough to start the engine, when the engine is warmer the track resistance is greater and the spark less strong. This problem gets worse over time as the carbon track gets bigger and bigger, the only remedy is to change the coil or electronic ignition module.

    Smoke signals

    For the most part, smoking is just as bad for a small engine as it is for you. Excessive smoke from the engine may be an indication of problems with the carburetor, rings, or gasoline:

    Lawn mower smoking after oil change

    (From: Wild Bill (

    I'm assuming that you were trying to use the drain plug at the bottom of the engine, not where the oil is added. Draining is done most effectively when the plug on the bottom is removed and the mower is placed level again, over a catch basin and left to sit.

    Oil has gotten into the combustion chamber area. The muffler (if it now contains oil) can be washed (flushed out) in a safe solvent, and allowed to dry.

    When tipping a vertical shaft engine to get to the drain plug, keep the sparkplug end of the engine higher than the rest of the engine.. and follow the safety precautions.

    Tipping the crankcase end up will put oil at the combustion chamber end of the engine, and seep into the combustion chamber.. and sometimes through the crankcase passage into the carburetor area. In cases where a lot of oil gets into the combustion area.. the engine might not rotate (due to a hydraulic lock), until the oil has returned to the crankcase.

    If this should happen.. allow the engine to sit for an hour or so, with the sparkplug end elevated, and most of the oil will return to the crankcase.

    If the air/fuel intake area has become flooded with oil, you might need to have the engine serviced.. the carburetor might need to be removed to evacuate the oil from that area.

    When the oil gets into the combustion area, the sparkplug is usually soaked.. and after cleaning or replacement, the engine will smoke (for a short time) like a fog machine. This will often foul the plug again, and create a lot of carbon in the combustion area. There isn't an effective way of removing the excess carbon aside from removing the cylinder head. For an old mower, that might not matter much.. for a new one that you'd like to get years of trouble-free service from, you might want to consider having the head removed and the oil & carbon cleaned out.

    About squeals and other animal noises

    While some may describe the engine of an antique automobile as 'purring', this will not likely apply to most gasoline powered lawn mowers. It would seem that noise reduction is just not a high priority design issue with lawn mower engineers or marketing types. However, even if not exactly quiet, the sound made by a healthy mower should not be similar to that of a pig being tortured.

    A combination of the above are also possible. For example, a loose flywheel could result in it scraping against the magneto yielding a sound like a cat being squeezed to death (or that of a first year violin student) but possibly only at high revs :-).

    Of course, a badly worn engine can also result in piston and rod slap and other mechanical noises as internal parts with excessive clearances whack one another. A complete engine overhaul may be in order or just tolerate it and plan for a new mower when the final day arrives (or your neighbors take up a collection).

    Lawn mower fuel tank leaking

    Fuel tanks can leak for a variety of reasons including defects in manufacturing, abuse, corrosion, etc. However, before you buy a new tank, a couple of notes:

    1. If this is on an Briggs and Stratton engine there are known problems with some models - check with your local small engine repair shop as it may be under a (hidden) warranty.

    2. A tank that is filled to the top may appear to be leaking when in fact it is just seepage from the cap vent hole. It may even attempt to fool you into replacing the tank by appearing to come from the seam!

    If the tank is truly leaking, DON'T use the mower and drain the gas - you don't want to take chances with a possible engine fire or worse.

    Lawn mower too loud

    Small engines - especially those on cheaper mowers - are usually loud, no question about it. However, if yours sounds like it is about to explode or take off, there may be something actually wrong.

    Obviously, if it is backfiring every other stroke, you have a problem with the ignition timing, mixture, valves, etc. What this section deals with is just the normal noise assuming the engine runs properly - and how to reduce it. Else, you need to perform the proper maintenance first.

    Note that for a 2 stroke engine, the muffler is particularly critical for proper operation and substitution may be more difficult.

    Lawn mower wheels

    Wheels tend to get banged about and damaged or may just become loose and unstable due to wear. Wheels and wheel bolts are readily available at home centers (or Sears for Craftsman mowers).

  • Back to Small Engine Repair FAQ Table of Contents.

    Intermediate Level Maintenance and Repair

    Cleaning Craftsman (Tecumseh) carburetors

    Simple float carburetors are found on a variety of equipment including many engines made for Sears by Tecumseh. The basic procedures applies to the float carburetors of other manufacturers as well.

    If you have been following the recommended preventive maintenance procedures, this may never be needed. But, face it, you do not! The most important PM that is not likely done by 90 percent of mower owners is to drain the gas at the end of the season. With float type carburetors in particular, the result is a buildup which eventually clogs the very fine passageways in the carburetor. What happens is that the gas in the carburetor bowl gradually evaporates leaving behind the gunk and varnish. New gas then flows in from the fuel tank which then evaporates leaving behind more gunk and varnish, and so on and so on and so on. This eventually, well, gums up the works by interfering with float movement and clogging the precision metering holes. Thus, the need for cleaning. Symptoms include difficulty in starting, flooding, surging, lack of power, difficulty in restarting when hot, etc.

    The following procedures are specifically for the common non-adjustable carburetors used on the vast majority of Craftsman mowers manufactured in the last 10 years. Carburetors with adjustments and/or a choke are slightly more complex and may differ in other ways. Refer to a small engine repair book or your engine manual for further information.

    Carburetor removal

    The carburetor can now be moved to the convenience of your workbench.

    WARNING: there is still likely a significant amount of gas inside the float bowl. Initial disassembly at least should be done outside so that you can dispose of this safely. Working outside is advisable in any case as the common carburetor cleaning solvents are both flammable and bad for your health.


    Most carburetors on Craftsman mowers are variations on a common float design. Newer mowers tend to have no adjustments and no choke - which greatly simplifies cleaning and adjustment. With respect to adjustment, there is none - it either works or it doesn't. If it doesn't, your cleaning was not thorough enough, some parts need replacement, or the problem is not in the carburetor.

    IMPORTANT: Before removing any linkages or springs, make detailed diagrams as to how everything goes together. You will NOT remember this several hours later!!! Reassembling the linkages EXACTLY as your found them is critical to the happiness of your engine.

    Turn the carburetor upside-down.

    Check the throttle plate for free movement - there should be absolutely no hint of binding or tightness. If there is, then this will need to be disassembled as well and cleaned:

    Cleaning the carburetor parts

    Use carburetor cleaner and lint free cloths or paper towels to remove all built up brown or green gunk, varnish, and other contamination from the metallic parts. Pay particular attention to the machined passages and metering holes.

    WARNING: Carburetor cleaner is both flammable and the fumes are toxic. Do all cleaning away from open flames and outdoors if possible. Wear eye protection. The stuff will also eat plastics including some plastic eyeglass lenses.

    Take care where non-metallic parts are still in place as extended contact with harsh solvents may degrade their properties (inlet seat and primer bulb, if present). Low pressure compressed air may be used to blow out passages but only use this on the fuel line from inside the carburetor body - else you may end up with the inlet seat clear across the driveway never to be found again.

    DO NOT use wires or metal instruments to clear any of the passages as their size is critical.

    The small hole in the hollow bolt on the bottom is most critical. Make sure it is cleaned down to the shiny brass and that this hole is unblocked and fully open:

         _     _
        | |   | |  
        | |   | |  
        | |   |_|  
        | |    _  Hole in nut (approximately .025") - use carburetor cleaner
        | |   | |  and wooden toothpicks to clear it out down to shiny brass.
        | |   | |  DO NOT use metal wires!
       _| | _ | |_
      |    \_/    |

    If you are absolutely sure there is no hole in the bolt (some models may forgo this), check further up on the central tube - there may be a tiny hole about 1/2" from the bottom. There has to be a hole somewhere for the gas to be sucked up through the carb!

    I first use carburetor cleaner inside and out with cotton swabs to remove all traces of gunk from the inside. Use as many as needed till no more discoloration shows up. Then, use the broken end of a wooden toothpick or popsicle stick to clear the .5 mm diameter hole in the side. In severe cases, this hole may not even be immediately visible due to the varnish and gunk buildup.

    If this hole is narrowed or clogged, the engine may start but then die in a few seconds. Gas enters the reservoir in the nut slowly or is forced in by priming but the normal suction cannot replenish it quickly enough.

    Fine steel wool may be used on the float hinge pin if it is rough or there is evidence of rust but do not use anything abrasive on any of the other parts. Persistence with carburetor cleaner and cloths or paper towels should prove sufficient.

    Inspect the inlet needle and seat. The needle should have sharp uniform edges and no visible damage to the conical tip. Any damage half way down the conical part - where it actually contacts the seat - will result in leakage and flooding. The seat can be removed if damaged by pulling it out with a hooked wire - careful - you do not want to scratch the body! If removed, do not reuse but install a replacement. The new seat goes in groove side first (lubricate with a drop of oil) and can be pressed home with a blunt rod.

    If the throttle plate was disassembled, clean these parts with carburetor cleaner. Use a cotton swab to get into the bearing surfaces in the carburetor body.

    DO NOT attempt to disassemble the carburetor beyond this point - the pressed in main fuel nozzle is precisely fitted and is not removable. The welch plug (pressed in disk) should not be removed unless you suspect contamination in the primer chamber (if any).

    Carburetor rebuild kits are available and are economical where almost any parts need replacement.

    (From: Jim Williamson (

    If soaking the carburetor in cleaner:

    When you remove each part from the dip tank rinse it with warm/hot water (as hot as your hands can reasonably stand). The parts that have passages - force water through the passage. This does two things: (1) rinses the internal passage of the cleaner and any old junk (2) gives you a VISUAL check that water is coming out the other end of the passage. The visual check is the key here - you could use compressed air to rinse the passages but you don't see the exit stream. On a clean passage the exit stream will be nice and solid indicating no particles hanging up in the passage.

    Now as for the hot water - this is to help dry the parts off - evaporation. Sometimes once I've rinsed the parts off I'll use compressed air to further dry the passages - or at least manually blowing through them.

    Critical considerations for proper operation


    Once all parts have been cleaned and inspected - replaced where needed, proceed as follows:

    If you removed the throttle assembly:

    Confirm that the throttle plate moves freely between a fully closed and fully open position - there should be no hint of binding or stiffness.

    Now for the main event:

    You can test for proper operation using low pressure compressed air (i.e., by blowing into the fuel hose), or water or gas. Water is safest but you must make sure to dry everything thoroughly before final assembly. To do this, temporarily reassemble the bowl with the hex head bolt. With the carburetor upright, dribble water into the fuel hose until it accepts no more - perhaps an ounce or two. There should be no leakage - the level of water in the hose should not change at all once it stops. If there is any leakage, there is still a problem with the inlet needle or seat - or the float is gas-logged.

    Carburetor installation

    With the carburetor positioned in its approximate location on the engine:

    Throttle/speed control linkages on Craftsman/Tecumseh engines

    The following is for one model! NOT ALL ENGINES ARE SET UP THE SAME! It is best to consult your engine manual. Getting it wrong is not something you really want to do! :(

    The user speed control (if any) pulls on a spring which is attached to the 2nd hole from the top on the governor lever. For engines with no speed control, there is a fixed plate or tang to which the spring is attached. Some amount of adjustment is possible by bending this plate.

    The carburetor throttle plate has several holes in it. The one that is probably used is that closest to the little cutout (which I can't show with ASCII art) and the tip of the throttle plate return spring. You can probably confirm this by looking for which hole has the paint worn off!

                         __                              _
              Open <-- /    \ --> Close                 | |
                      |o=========.               ,=======o| Top hole
                       \ O   /    '============='       | |
                         \_/ Carburetor   +-- ===========o| Next hole
                 _           Throttle     |             | |
                |o|          Plate        +-straight-+  | |
                | |                                  |  | |
                |o=====o---/\/\/\/\/\/o========== ---+  | |
                | |           Spring                    | |
               | O |                                    | |
             Speed Control                              | |
             or Fixed Tang                    Governor  |_|
            (linkages may cross)                 Lever | O |

    See: Neil's Tecumseh Throttle and Carburetor Linkage Page for some slightly better diagrams. :)

    Initial tests

    Add a small amount of gas to the fuel tank - perhaps half a glass or so. Just enough to assure that it will reach the carburetor even if the mower is slightly tilted or jostled.

    Inspect around the fuel hose and carburetor body for fuel leaks. If gas starts dripping from the air inlet or anywhere else, there is still a problem with the inlet needle and seat. Disassembly will be required.

    Only a few seconds are needed for the gas to fill the carburetor bowl.

    Assuming there are no leaks, install the air filter and reattach the spark plug wire or reinstall the spark plug. Attempt the normal starting procedure - prime if recommended.

    The engine should start on the first pull! Immediately move the throttle selector to LOW if you have this option. Confirm immediately that it stabilizes at a reasonable speed - stop it quickly if it sounds like the mower is preparing for takeoff - your governor connections are incorrect or binding. If it runs at a fast speed with the speed selector set at LOW, the governor spring is probably in the wrong hole. Check it.

    Listen and feel for any significant unevenness, surging, or other unusual behavior. Stop the mower, wait a few seconds, and restart. It should restart with a single pull without priming.

    Mow for a few minutes. Stop the engine and confirm that it restarts without priming. Listen and feel for any indication of lack of power or other unusual behavior.

    Go take a dinner break. Then confirm that the engine will now start - priming may be needed since it will now be cold.

    Maintenance of point-type ignition systems

    In most cases, missing or total lack of spark will be due to dirt, pitting, or corrosion of the points or a failure of the condenser. Timing may affected as well by excessive wear. The following procedures should restore the ignition system to good health:

    First, test for spark. If there is a spark, try replacing the spark plug since this is the most likely cause of ignition problems. With a spark present, there could still be ignition system problems but this is much less likely.

    You may be able to test the points to some extent if you can get to the wire that connects to the magneto or the STOP switch. A multimeter on the low ohms scale will the permit you to watch the opening and closing of the points.

    If this confirms that the points are operating the condenser could still be defective, the breaker arm could be sluggish, or the point gap could be grossly out of adjustment.

    To proceed further:

    Setting the point gap and ignition timing

    Perform the following whenever the breaker points assembly is replaced or where a timing problem is suspected.

    Recoil (pull) starter repair

    WARNING: Take extreme care should you need to disassemble the portion of the starter that includes the mainspring - that can be nasty.

    Broken lawn mower handle

    Should the tubular handle on your mower break at some point (yes, I know this should be unlikely but I know of two people who have managed to do it!), the use of a splint is probably the best approach. Obtain a length of steel pipe with an inside diameter just a hair larger than the outside diameter of the handle. About 8 to 12 inches should be enough. Even electrical conduit may work. Use this to splice the joint. Drill holes through both the pipe and handle and secure with sheet metal screws. This will maintain the required strength and keep the handle the same length as it was originally so you will not have to become a midget to mow your lawn.

    Diaphragm carburetor problems

    "I Recently inherited a BobCat Snow Blower from the 60's or 70's. It has a Lausen 3.5hp (HR35P-2403P). I just installed a Tecumseh Rebuild Kit#631893. The unit runs way too rich, I run out of gas in 10 minutes, the muffler starts to glow red. I cannot get it to idle. I installed a new needle, seat/jet and diaphragm. During the rebuild I did not remove the welch plugs. I tried swapping the idle screw with the high speed screw (not sure which is which). One screw has a smooth taper while the other has a taper with a step to it (no evidence of scoring). Could I be missing a key piece that regulated fuel flow? The rebuild kit did not come with directions, so I just installed everything in reverse."

    The idle mixture screw is the one with the step.

    I assume you have any choke off, throttle connected properly with spring return, etc.

    What happens if you close both idle and main jets? Does it starve? I would expect that you should be able to stop fuel flow totally. If this is not possible, your needles or something else is incorrect/worn or fuel is somehow bypassing the jets which is also wrong.

    Did you compare the old and new parts to make sure they gave you the correct kit?

    It also recommend removing the Welch plug and blowing compressed air through the passages to clean.

    It might also be a problem in the diaphragm spring pressure but without seeing it, no way of knowing. The diaphragm acts against atmospheric pressure. There is a spring on the inlet needle which if missing would run very rich. Chilten has a detailed diagram - really no way of knowing if your assembly was done correctly. Also, warns again using harsh cleaners on non-metallic parts and clearing all vent holes.

    However, I rather suspect that comment about running rich is not correct as an engine running very rich would lack power if it continued to run at all. Your throttle may be stuck wide open and it may be over revving.

    Your public library should have some Chilten or other books like those listed in the section: References. These should include diagrams of the diaphragm type carburetor.

    Metal gas tank repair

    Yes, you have to remove the gasoline first! And, make sure all traces of vapors are gone - if it smells like gas, it may very well explode! However, this isn't as much of a problem as with an automotive gas tank.

    (From: Greg Fretwell (

    The best way is to braze it, but before you get your torch out take a bottle of ammonia to the car wash and have them steam the tank out, pour in some ammonia and rinse. Repeat a few times. Ammonia will remove the residual gasoline and make it safe to weld on. This is the procedure required by the welding shop that fixed my tank. They needed to see the receipt from the car wash before I could even bring the tank on the property.

    (From: Doug Younker (

    I have gotten lucky and have been able to solder repair leaks in small engine tanks using my 140 watt soldering gun after leaving the tank to preheat in the sun all afternoon while I was at work. You have to be patient to do so...

    (From: Jeremy (

    If you want to work on a metal gas tank, empty the gas and put some dry ice in the tank. The dry ice will vaporize, filling the gas tank with carbon dioxide thus removing oxygen and gasoline vapors.

  • Back to Small Engine Repair FAQ Table of Contents.

    Engine Overhaul

    When does this information apply?

    This chapter deals with the following:

    For detailed instructions on valve grinding, cylinder reboring, or main bearing reaming, for example, you should refer to one or more of the books listed in the section: References. However, this chapter will give you the general feel and basic information needed to perform many common types of simple overhaul operations and to evaluate the need for more drastic action - such as a trip to the new lawn mower store!

    Depending on your particular problem(s), only a subset of these sections may apply. For example, inspection and cleaning of the valves and combustion chamber - even valve regrinding (but we said we weren't going to talk about that!) can be done with a minimum of engine disassembly.

    Do you need an overhaul?

    Many common problems can be remedied without going into the deep dark recesses of the engine. However, some will require either a partial or total overhaul. Eliminate all other possibilities from consideration before considering an overhaul - it will not be a fun afternoon (or weekend, or week, or month,....).

    The following are indications that at least a partial overhaul may be needed:

    In some cases, multiple problems may be present and/or there may just be excessive wear of parts like the cylinder, rings, and piston. Under these circumstances, the cylinder may need to be rebored to accept a replacement oversize piston and ring set. The cost of the parts and labor (you really don't want to rebore a cylinder) will likely be more than you want to spend. This is when a new engine or mower is the best option.

    Comments on engine rebuilding

    While the specific question dealt with a medium size snowblower engine, the comments should apply to other yard equipment as well.

    "Is it economical or feasible to properly rebuild a 7 HP Tecumseh engine on a snowblower? Compression seems fine. Has been burning oil to some degree for the last 3yrs, but this year its' burning a lot - maybe 1/2 pint oil for each gal of gas. Until last year, was using 5W30. This year, switched to straight SAE30. I could get a new Tecumseh SnoKing engine for about $350 including shipping."

    (From: Mother (

    As a finalist in the All-American Engine Repair Championships formerly held at the Outdoor Power Equipment EXPO (an industry trade show) in the Tecumseh division, and as a Briggs and Stratton Master Service Technician, it has been my experience that:

    1. Yes it is POSSIBLE to rebuild one successfully, although if it is not an HH model with cast iron bore, it probably will not hold up (single H models are aluminum bore).

    2. It is not cost effective to do so.

    3. Short blocking this engine requires special tools if it is more than ten years old, as the ignition timing is not fixed, as it is on newer, solid state models.

    4. Engines from companies like Northern Hydraulics may be adaptable to your unit, but will likely not just bolt on.

    If this engine is on a top of the line product, such as Ariens, Snapper, or BearCat, it is probably worth repairing, as a new comparable product is big bucks (and overpriced).

    If this engine is on a mid-range product, such as Toro, Simplicity, John Deere, Husqvarna (European product, not USA built), etc., then repair is probably still a good option, due to the overpricing of similar replacement products.

    If it is on a Murray, Noma, AMF, Dynamark, Ultra, Sears, MTD, YardMan, White, Husqvarna (USA built by Murray/Noma), or other discount store brand, go buy a new machine. The cost of a new unit is not much more than the cost of the engine repairs, and then you won't have a worn out piece of discount store equipment to break down again in three weeks when something else goes bad...

    Compression testing

    Special compression gauges are available at auto parts stores or small engine parts suppliers. These will catch and hold the highest pressure reached so you don't need to be in two places at once.

    Note that this procedure may always yield a very low reading if there is a compression release mechanism on your engine - which is very likely. In this case, the crankshaft must be spun in the opposite from normal direction by the flywheel (counterclockwise when viewed from the flywheel end, with the starter removed).

    For Briggs & Stratton engines, the manufacturer simply recommends spinning the crankshaft by the flywheel in the opposite from the normal direction with the normal spark plug installed. A sharp rebound (as it compresses the trapped air since the compression release mechanism is not active in reverse) indicates good compression. Little or no rebound indicates low compression and need for service.

    Should you even bother?

    Only you can decide if the time and effort will be worth it. If you enjoy a challenge, then engine overhaul may be for you. However, this is probably not going to be your idea of fun. Doing something like this for the first time *will* result in scraped knuckles and the liberal use of 4 letter expletives. If the mower was a Hechinger's $100 special and has seen several seasons of use, then it may be time for a new one.

    If you are not the detail oriented meticulous type, you may be better off leaving this sort of overhaul to a professional or buying a new engine or mower. Most parts must go back in exactly the same orientation as they were originally - including matching of timing marks on the crankshaft and cam gears. Even the piston is not symmetrical - though this is not obvious except by taking detailed measurements. Nonetheless, it will not work well if at all, or will wear quickly if rotated 180 degrees upon reassembly.

    Furthermore, once a wear pattern has developed, it is generally a good idea to replace parts in exactly the same position - the direction of the piston (wrist) pin or location of the intake and exhaust valve lifters. Violating this rule won't result in immediate failure but could lead to excessive wear and reduced life

    What this means is that you cannot assume anything about the parts you remove. Even if they look identical at first glance, they may have a definite right and wrong orientation and/or may want to be replaced in exactly the same location. Even lowly head bolts may be of different lengths. Make notes and diagrams. Most of these will be pretty simple but they will save your hide in the end!

    Bearing surfaces are very finely ground and polished - just dinging the crank pin journal surface against a steel part will put a nick in the relatively soft bearing which will need to be carefully removed as best you can - affecting as little else as possible - with very fine emery cloth.

    In addition, if you don't like to get your hands dirty and oily, forget it. You will have disgusting black crud under any surviving fingernails for days. This is a messy operation! The outside of the engine will be coated with decayed grass clippings, dust, and dirt. The inside of the crankcase will have the remnants of old used motor oil (also a carcinogen) and the combustion chamber will be coated with filthy carbon deposits.

    At the same time, cleanliness is critical when reassembling as any particles of dirt or metal will find their way between rotating parts resulting in excessive wear or worse.

    Having said all that, overhauling a small engine is not like overhauling an automobile engine. In the words of a colleague: "It's just a frick'n lawn mower". What this means is that you can get away with tolerances, imperfections, and mistakes in dealing with a small engine that would be unacceptable for the health of your Chevy or Porsch.

    Special tools

    For most of the procedures described below, the basic set of items listed in the section: Tools and supplies will suffice. However, some of the following more specialized tools may be needed depending on how far you go:

    Prepare your work area

    Once the gasoline and oil is drained, all overhaul work can be done indoors. Without gas, there is little risk and working indoors is generally much more convenient. Therefore, you need to decide where to set up for the overhaul. The best location will be relatively dust free, well lit, and not likely to be required for other purposes. Your engine may be remain in a disassembled state for some time if you need to obtain replacement parts. Also, even with its bodily fluids removed, an old engine will stink. Therefore, the dining room table is probably not the ideal choice!

    Lay out a healthy layer of old newspapers to protect the workbench from oil and solvent drips and damage from heavy tools and parts.

    Engine overhaul procedure

    The following description applies directly to a large number of Craftsman mowers using Tecumseh engines (most do). However, with minor modifications, it is also applicable to most other mowers using 4 stroke engines.

    Mechanically, 2 stroke engines are very similar. In many respects, they are simpler having no camshaft operated valves or oil pump. There is no oil to drain or change. However, needle bearings are used in key spots which complicate matters slightly. Refer to one of the books listed in the section: References for detailed 2 stroke overhaul procedures.

    Prepare the engine for removal

    While for certain repairs it is quite possible to work on the engine while still mounted on the mower or other yard equipment, it will almost always be much more convenient to disconnect and remove the entire engine to the convenience of your workbench. There are generally only a handful of actual connections. A typical small engine is remarkably light and compact once stripped of the mower deck!

    Remove the engine

    At this point, the engine should be free of all its attachments to the mower except for its mounting. For a typical rotary mower, there will be three large bolts accessible from under the deck. Removing these with the proper socket will allow the engine to be lifted and moved to your workbench. You will probably be surprised at how light it is! I recommend just screwing the bolts back into their threaded holes finger tight. That way they will not get lost and the threads will be protected. Also, Protect the threaded end of the crankshaft with a bit of rag or paper towel fastened with an elastic band.

    Engine disassembly

    The following sections provide the detailed procedures for disassembly and initial inspection for major damage. As noted, these apply directly to most Tecumseh engines but most other 4 stroke engines are very similar.

    Removal of the accessories

    Now it is time to get down to business! As noted, depending on your situation, not every step will be needed.


    The carburetor can now be set aside or disassembled and cleaned. (See the section: Cleaning Craftsman (Tecumseh) carburetors.

    WARNING: there is still likely a significant amount of gas inside the float bowl. If turned on its side or upside-down, this gas will come gushing out. Therefore, it is best to set the carburetor aside in an outdoor area in an upright position. Plug the intake manifold and fuel pipe with wadded up paper towels or rags to prevent the entry of dirt.


    Note that in addition to decreasing the noise from your engine, the muffler serves a very important spark/flame arresting function. Therefore, it is important that it be in good condition.

    Some mufflers simply screw into the cylinder using pipe threads. Others are mounted with a couple of bolts.



    This can be left in place but will be susceptible to damage.

    Cylinder head


    You should only need to do this if you are replacing or grinding a valve. In most cases, the valves are undamaged but may appear in poor condition due to carbon buildup - which can be removed in-place fairly easily.

  • Remove the valve cover on the side of the engine next to the valves.

  • Use a feeler gauge to check the gap between the valve lifter and valve stem with the valve lifter in the relaxed - lowest - position. A typical value is .010 inches. Excessive clearance will require replacement of the valve or valve lifter. There could also be too little clearance - which is also bad - but the valves stems can be ground down to repair this problem which is either a defect in manufacturing or a result of a cam shaft replacement which is oversize.

  • Use a valve spring compressor to remove pressure on each valve.

  • The retaining clip or split cup should now be free. Remove these.

  • The valve should now slide out.

  • Loosen the valve spring compressor and remove the spring and any other hardware. Don't interchange the intake and exhaust valve springs.

  • Remove the built up carbon deposits from the valves and valve seats using a soft metal scraper. Some WD40 may help to loosen the caked on carbon.

  • Inspect the valves for serious burning or pitting especially on the seating (angled) surfaces. Once free of any carbon, they should be smooth and undamaged.

  • Similarly, inspect the valve seats for serious burning and pitting.

  • Inspect the springs for rust or other damage. Stand them on a flat surface and check for serious droop/tilt. Measure their free length and compare with your engine specifications.

    (From: Brian (

    On old 3.5 and 4 HP Briggs and stratton engines the cam acts weirdly and opens the exhaust valve slightly on the compression stroke thus loosing air and fuel mixture and compression. This is probably due to wear of the valve or valve seat resulting in the cam pushing the valve up when it's really not supposed to be touching the tappet. You need to pull the head off and crank it slowly and watch the exhaust valve, if both valves close and then the exhaust valve pops up, an adjustment is required. To fix this grind the exhaust valve tappet side down 0.5 MM put the valve back in and check it out if it still does grind a little more off. the engine will run smoother than before and a little better

    (From: Sam.)

    Note that many small Briggs and Stratton (and other) engines include a compression release feature to make starting easier. This has a similar effect in that it opens the exhaust valve during part of the compression stroke reducing the effort needed to pull the starter rope. This should not be confused with what Brian is describing, above. At normal operating rpm, the governor moves a secondary piece on the exhaust valve cam out of the way so that the behavior returns to normal. Unfortunately, unless the crankcase is open, there is no easy way to determine whether the compression release is present on your engine without a service manual.

    Oil sump/crankcase cover

    We are now going into the lower section. I can hear you saying "Joy!".


    Connecting rod




    Oil seals

    Detailed inspection

    A complete overhaul can restore a small engine to like-new condition. Any parts that are found to be damaged or out of tolerance are repaired or replaced.

    Determining this requires a visual inspection and taking measurements of all critical dimensions of bearings, cylinder, piston, and rings. Some of the inspection is subjective - how badly scored a bearing surface is before it must be replaced or reground. A few score marks around the circumference of a bearing surface will not adversely affect operation or wear. How few is a few? Perhaps if less than 10% or so of the surface is affected. You are not going to spend as much to repair the mower as it cost in the first place in any case so don't lose sleep over it.

    Measurements may come up marginal as well. For example, if the limit listed in your engine specifications is .0015" and you measure .002" will this be a serious problem requiring the replacement of expensive parts? Probably not. You may get less than optimal life out of the engine but it will probably still work fairly well and for a long time. So many other factors can affect life that this may have no effect at all.

    The following items should be visually inspected. If any significant wear is indicated, precise measurements should be made:

    Engine reassembly

    Once you have performed whatever magic is required to repair or replace broken or damaged parts, here are the steps that will transform your pile of parts into a (hopefully) working engine.

    If any filing, sanding, or grinding was involved, make sure all traces of abrasives have been removed from every part. The best approach is to clean with soap and water or mild detergent and dry thoroughly. Then immediately coat all ferrous parts with engine oil to prevent rust.

    Where the internal moving parts are involved, liberal use of fresh engine oil will also make things to go together smoothly and help protect the surfaces from damage due to initial lack of lubrication.

    For the following, position the crankcase flywheel/magneto side down on some wooden blocks so that when the crankshaft is installed, it's end will be clear of the table-top.

    For the following steps, position the engine on wooden blocks blade/PTO side down.

    Engine installation

    It will be easier to tighten the flywheel nut to the recommended torque once the engine has been reinstalled on the mower. Therefore, now is a good time to install the engine to the mower deck:

    Initial post-overhaul testing

    Assuming you didn't make any mistakes, the engine should start on the first pull. As you start it, look and listen for any abnormalities and immediately stop it if any are detected:

    Assuming nothing appears wrong, run it for a while at slow speed (if you have the option). Continue to be on the lookout for anything unusual. After a few minutes, stop it.

    Let is sit for 10 minutes or so and then check, and if necessary, top off the oil.

    Now, restart and run it at high. Mow a few lawns.

    Congratulations! Hopefully, your engine will now serve you for many more years - or until the blade hits the next curb!

  • Back to Small Engine Repair FAQ Table of Contents.

    Items of Interest

    Explosion risk when filling a metal gas can

    Although rare, there have been reports of fires resulting during fuel transfer. Simple precautions can minimize this risk.

    Here is how to fill a gas can to minimize the danger of fire:

    More information can be found on: Chevron's Technical and Safety Publications Page.

    Why you really don't want to attempt to move an immovable object

    The rotating blades and mass of the internal engine parts pack quite a punch. Speeding along merrily mowing away one doesn't think about this. However, if the blade should hit an obstruction, you may have no choice.

    There are various safeguards to protect the mower from damage should a blade tip hit something but these don't always work. Why?

    There is protection for the upper and lower parts of the crankshaft after all:

    In many cases, both of these will break free at the same time.

    However, if the shock is severe enough, much more serious damage can result. Here is why: When one end of the blade hits a curb, for example, the inertia of the mass of the blade alone (rotating at high speed) will attempt to push the shaft sideways. This is pretty much independent of the rest of the engine.

    In the diagram below, the blade is rotating clockwise. When the left-hand tip hits the curb, the right-hand side due to the inertia of the entire right-hand half of the blade wants to continue to move (down in this diagram) with the 'X' as the fulcrum. The entire left-hand half section of the blade contributes relatively little. This results in a net significant sideways (downward in the diagram) bending force on the crankshaft. The unavoidable arrangement of the fulcrum at one end and the shaft in the middle makes the situation even worse as the force resulting from the blade tip (the right hand one in this example) is amplified by the up to 2:1 mechanical advantage of the lever arm (the tip is twice as far from X and the shaft).

        Blade tip   /
      hits curb.  X-------____________________________________
        Ouch!   / |                                           |   Inertia of this
              /   |                   - O -                   | | side of blade
            /     |____________________________________       | | attempts to 
                                        |   |   |   |  -------  | continue in same
                                        V   v   v   v   v   v   v direction.
                                Net sideways force
                                  on crankshaft

    While the rotating mass of the engine is attempting to shear the blade lock key, the inertia of the blade is trying to push the crankshaft sideways. The net result could be a severely bent crankshaft - a very expensive repair. An 8 to 10 degree bend is not unusual for a typical Craftsman-class mower running at full power. Any detectable bend in the crankshaft requires replacement - it is not safe to attempt to straighten it. A bend resulting in the blade tips wobbling by more than a fraction of an inch, there will be unacceptable and dangerous vibration when the mower is run. In addition, the original trauma (as well as attempting to run with a bent crankshaft) can damage other parts like the main bearings and connecting rod. The blade lock and flywheel keys will likely be broken as well but these are insignificant in comparison to the cost of major replacement parts and the labor involved in their installation.

    The time and effort needed to disassemble the engine is significant and the crankshaft is probably the single most expensive part of the engine. In fact, purchasing a new crankshaft may be more expensive than an entire new lawn mower! It is quite possible that unless you have access to low cost replacement parts from a salvage yard and have the free time to do the work, repair may not make sense.

    Therefore, don't let this happen to you. Your curbs and rocks don't grow that quickly and do not generally require mowing!

    How about a crankshaft friendly blade?

    It is too bad that most lawn mower blades are made of heavy rigid steel (though I do recall a mower that used a super thick nylon cord - sort of like a weed whacker on steroids). The chance of serious engine damage from curb kissing would be greatly reduced if a blade were used which had less inertia and increased flexibility. Then, no matter how hard you tried to whack something solid, only the blade lock and/or flywheel keys would shear and thus spare the expensive engine parts.

    Apparently, some mowers are made with swing tip blades:

    (From: Roderick Carmichael (

    "I use a top notch Supa-Swift with a gravity cast alloy chassis and swing-tip blades (Australian invention, no bent cranks on our mowers mate!)"

    Why are these not common in America? Conspiracy to sell replacement parts by the small engine manufacturers? :-). You would have to really work at bashing a curb to bend a crankshaft with such a mower.

    Another possible approach - applicable for both new lawn mower designs as well as field upgrades - would be to replace the heavy steel blade with one made of nylon with a molded-in steel edge. An auxiliary flywheel might have to be added (under the deck) to provide the needed inertia (normally supplied by the steel blade) for the engine to start and run properly and to help the mower plow through tall grass. However, since this additional flywheel could never be stopped abruptly due to hitting a rock or curb, its inertia would never contribute to a sideways bending force on the crankshaft. The greatly reduced mass and increased flexibility of a reinforced nylon blade should virtually eliminate the possibility of a bent crankshaft from such unfortunate incidents. See the section: Why you really don't want to attempt to move an immovable object.

    (From: Mowerman (

    I have been reconditioning lawn mowers for many years and find it fascinating what design goes into some of this equipment. Like arrangements to prevent the crankshaft from bending if the blade hits something. Most engines have an aluminum key at the flywheel but this will not stop shaft bending most of the time. Mower manufacturer uses many methods. Lawn boy has a tapered shaft at blade level and so the shaft has no key and the blade slips if hit. Some other mowers had steel washers and fiber washers nearest the blade like a clutch I seen this on some antique mowers. The best idea is disc with small flail blades or short blade with flail at end. Manufacturers in New Zealand and Australia use this method on their domestic mower mostly and it works well. Victa of Australia was one of the first ones around here with this idea. I am sure you have them over there to no doubt. I hope this information will be to some help.

    Crankshaft-friendly blade sighted

    (From: Thomas Prufer (

    I recently came across a description of a blade assembly similar to the one suggested by you to make bending the crankshaft difficult. This is in the Royal Horticultural Societies' "Encyclopedia of Gardening". In the tools and accessories section, under the "lawn mowers" subsection, a caption says: "Plastic disk. This cheap replacement part cuts the grass by rotating horizontally". The inset shows rotary, cylinder, and "Flymo" mowers. A picture of the part shows a disc almost the diameter of a regular blade, with one blade joined to the circumference by a pin so that centrifugal force keeps it radial. This blade is short and narrow compared to the plastic disc; no telling if it is plastic or metal. There is no further reference to this part in the text. In my translation, it is on page 465.

    If only they had included a name or supplier!

    Another reason not to mow rocks!

    (From: Forbes Family (

    I have recently purchased a new rotary lawn mower and appear to have started wrecking it in the first two weeks of use! Problem is, my property has lawns that run alongside a gravel driveway, and its often very hard to guarantee there are no stones lying in the grass before you start mowing. Not surprisingly I often hit small stones. These usually cause no harm, but today some teenagers were mowing my lawns and hit a real monster that measured approximately three inches by two by one, and weighed more than half a pound! Although the mower seems still to work OK, the impact has created a three inch long tear in the mower's cast aluminum body. Not a nice thing to have happen to a new machine!

    Despite the stone's size, I was surprised at the size of the resulting tear in mower's more than 1/4 inch thick aluminum casing. I hadn't imagined a rotary mower blade powered by a 5 HP Briggs and Stratton motor could produce such force!

    It would be interesting to hear from others who have survived similar experiences and to get an idea from any budding engineers on whether its perfectly reasonable for a stone this size to do such damage to the body of my mower - or whether it's more likely the body casting had a manufacturing defect that made it split prematurely?

    Rod disasters - or why the oil and governor are kind of important

    A combination of low oil (well, actually, almost no oil) and probably too high RPMs resulted catastrophic failure of the connecting rod and cap on my garage sale Eager 1 Craftsman mower. I had just completed cleaning the carburetor and was testing it when p-ting!! and it stopped dead - the rod had broken and it was dead-dead. Extremely embarrassing since there was no excuse for such a disaster.

    The primary cause was likely a lack of oil - I should have checked it before attempting to run the engine for more than a few seconds. I have no idea whether someone had actually drained the oil for who knows what reason or it was just very low. In addition, I may have accidentally put the governor link back in the wrong hole permitting the engine to run at an abnormally high (and dangerous) speed.

    There was no warning. The rod cap just exploded into e pieces (and this was at normal speed) and took a nice chunk out of the interior of the crankcase. Based on a post mortem of the rod, it appears as though one of the cap screws just loosened and backed its way out totally - there was no evidence of thread damage that would be expected if it were ripped out - and fell into the sump. With only one screw holding the rod and cap together, eventual failure was inevitable. Due to the offset design of the cap, this probably worked for a while since most of the force is on the rod.

    Discoloration indicated excessive heating but no obvious bearing damage was evident that could be attributed to the lack-of-oil condition. The bearing was not in pristine condition but the type of scoring seemed to be more due to just poor general maintenance - lack of regular oil changes - than to this incident in particular.

    Lessons: Check the oil level no matter what if there is any question or you are working on an engine of unknown history. Double check the governor linkages - take notes during disassembly - and be aware of what a normal speed sounds like for your type of engine (2 stroke or 4 stroke). If in doubt, install the link in the hole that would result in lower RPMs - closer to the carburetor. You can always move it later.

    I forgot the oil and now it's stuck

    OK, so you didn't read this document first or just got distracted while changing the oil and you ran the thing with no oil until it came to a screeching halt. Assuming nothing actually broke - it just won't turn at all, there may still be hope without a complete refurb (which in reality probably means a new mower):

    (From: David Thomas (

    The following assumes it is (was) a small push mower with a vertical shaft engine and that you drained the oil the preceding fall.

    What little oil that was left in the bushing areas on the crank have burned down to tar. The piston may or may not be seized. Either way you'll need to break the engine down (pulling the flywheel is the hardest thing about it) and clean the tar out.

    If this is beyond your abilities, try spraying penetrating oil on the crank around the bushing areas both on top (may still have to remove the flywheel to get to this area) and bottom as well as around the piston as the other gentleman suggested. (If you can't remove the head, pull the spark plug and spray penetrating oil or WD40 inside the chamber using one of those straw things that comes with the can so that you can direct the spray around all the walls of the cylinder, put the plug back in so it doesn't all evaporate and then set the engine so the piston is pointed straight up and let it soak a few hours.)

    Last choice is to fill the crankcase with about a pint or two of penetrating oil (and spray inside the combustion chamber as stated above) then slosh it around the engine, turning the engine every which way and upside down, let it soak for a couple of hours repeating the sloshing every so often and changing the position of the engine so that you alternate soaking each bushing area and the bottom of the piston and then try the pull rope again. Be *sure* to disable the spark 'cause you *sure* don't want it to start with that light oil in the crankcase.

    If it does free up, drain out the light oil and add the normal recommended oil before trying to start it. If it starts (it will smoke like the devil while it burns that light oil), let it run at idle speed until it warms up then shut it down and change the oil. It may still burn oil and smoke since the piston walls are probably scored badly so check the oil every time before starting it even after a short break until you get a feel for how much its going to burn.

    (I had one engine that I did this with that used all most as much oil as it did gas. I had to clean the crud out of the spark plug before each mowing.)

    (From: Brian Fistler (

    If it's a Tecumseh engine, throw it away... Even if you get it running, it'll probably throw a rod soon (not like it wouldn't even if you hadn't seized it... :-)

    If it's a Briggs, try putting oil in it first, then remove the spark plug and spray a liberal amount of WD 40 in there... You might have to let it set for a day or so, then, LEAVING THE SPARK PLUG OUT try turning the engine using the blade... Once you get it freed up, most likely it's run fine and probably last a few more years...

    When I was a kid, my best friend and myself used to "work" on Briggs engines, we'd tear them down, fix broken ones from the junk yard, and most fun of all was "torturing" one... After seizing it up by finding out what would, and would not keep the thing running by spraying it in the carburetor (i.e., using hair spray, WD-40, paint thinner, etc...) We finally found that a certain spray engine degreaser of the era *would* run the engine, it would not run it for *long* because it removed the oil from the cylinder walls, and the engine would seize up...

    That gave us an excuse to tear the engine down and find out how to get it going again... After the 3rd or 4th time seizing the engine with various products, we just decided to cut a hole in the side of the block and put a piece of Plexiglass there, so we could get easier access...

    We ran that poor engine with every possible thing in the crank case, including pure water... (Side note: Water didn't seize the engine, as long as you didn't allow all of it to evaporate)

    When the engine would seize up, all we ever had to do was put oil back in the case, spray a little WD-40 in the spark plug, and hit the blade a few times with a hammer to get it starting to turn...

    That motor kept us entertained for an entire summer... :-)

    So you got oil in the cylinder

    "I tilted my Toro to work on it and now can't start it because oil flooded into the cylinder. I already cleaned up the plug. Is there anything I can do to clean out the oil without taking the engine apart?"

    Possibly, just letting it sit for awhile (in the normal position!) will allow the oil to drain back into the crankcase sump.

    If oil is really trapped between the piston and the head, then you may be able to just tip the lawn mower so that the spark plug hole is down (a buddy may come in handy) and drain the oil out through there.

    Alternatively, you should be able to suck most of it out with a kitchen baster and narrow extension tube (make sure it is made of something that won't scratch the interior of the cylinder and the piston) through the spark plug hole.

    You don't have to get every last drop. What is left should not prevent you from starting the engine - it will just belch gobs of white/blue smoke for a few seconds after it kicks over as the remaining oil burns off. Keep in mind that squirting a half an ounce or so of engine oil into the cylinder is recommended when winterizing to protect the cylinder from rust so it should not be a problem.

    In fact, I would expect that pulling the starter a few times will clear most of it in any case. It is possible that you have other problems - hopefully you didn't turn it over carburetor side down!. (In this case, the air filter may need to be removed and cleaned or replaced.) It may even be that your initial attempts to start it with an oil in the cylinder have resulted in a flooded the engine (excess gas) and waiting will clear that as well.

    Some of the following information may be model specific but most applies to any engine that has gotten oil in the cylinder and/or carburetor due to tipping:

    (From: J. Matthew Good (

    First, my guess is that it is a Briggs QUANTUM or SIGNATURE SERIES engine, with the paper air filter. Go buy a new filter, as that one full of oil is shot.

    Next, remove the plug and secure the plug wire away from the opening. Crank the engine a few times to clear the liquids out of the cylinder. Reinstall the plug. Now take a 1/2 inch box wrench and loosen (don't remove) the plug/nut on the bottom of the carburetor until gas flows clear through it, and retighten it. This should get the oil out of the carburetor. Now, check the oil.

    Since you lost so much into the carb, and air filter, and it only holds 2 and 1/2 cups total you will probably need to add oil. If not, you may have gas in the oil as well. Drain the oil into a pan for recycling by tipping the mower air filter up, and dipstick tube down. Fill with clean SAE 30 HD oil. DO NOT use 5W30, 10W30, 10W40, or any other W oil. Just SAE 30 HD from any discount store will be fine. Do NOT use SAE 30 ND, it does not have the needed detergents.

    Now you have the liquid out of the cylinder, the oil out of the carb, the gas out of the oil, the air filter OFF, and you are ready to start the engine. If it has a CHOKE, set it for full choke, if a primer, press it 3 times. Pull the rope until it starts. Let it JUST RUN until the smoke clears, don't mow or anything else until the smoke clears and you reinstall the NEW air filter.

    The reason I guessed it was a Quantum is that this is the only engine I know of that automatically puts oil in the air filter if you tip it for sharpening. That's why the first thing I do to any Quantum that comes in for service is remove the air filter and put it in a safe place.

    (From: Lloyd E. Sponenburgh (

    Actually, a judicious tilt *away* from the carb will coat the underskirts of the cylinder and piston with oil so as to make starting *easier*. This improves compression. It's an old salesman's trick to show just how easy it is to start the engine.

    So you put gasoline in the oil filler hole

    Don't panic, just get it out of there. Drain the oil/gas mixture completely and refill with the proper new engine oil. If you want to be doubly sure, run the engine for a minute or so to mix any of the remaining contaminated oil with the new oil, then drain and refill again. It really shouldn't have done any damage if you get it out of there reasonably quickly (I don't know what effect gas might have on the oil seals over time though).

    Oil starvation from mowing at too steep an angle?

    4 stroke engines require oil to continuously coat the various moving parts during operation. There are two types of systems used in most small engines: splash lubrication and forced oil lubrication.

    With the oil filled to the correct level (yet another reason to check the oil every time you mow!), the oil starvation angle should be greater than anything you are likely to safely encounter unless you have a very hilly lawn. However, if you do mow steep slopes for more than a few seconds (e.g., to turn around), it would be worth determining if this could be a problem for your engine.

    Two stroke engines do not have this problem since the oil is mixed with the gasoline. As long as the fuel feed is working, the engine should be happy and the mower will mow! However, I do not like 2 stroke engines because of their generally higher production of smoke and pollution.

    (From: Walt Conner (

    Oil starvation can happen. If you must mow on a very steep slope, look to see which side the valve cover plate is on. This side of the engine will also have a semi-circular bulge in the crankcase while the opposite side will be pretty well flat. The side with the bulge will have the oil slinger located below the bulge. Keeping this side down slope should keep you in oil anywhere you can stand.

    When failure occurs, usually the connecting rod is first to go, sometimes the top main bearing seizes. Either occurrence usually means it's time for a new mower.

    For really hilly mowing, you may want to consider a model that uses a 2 stroke engine such as Lawn Boy.

    Additional comments on winterizing - draining versus the use of fuel a stabilizer

    Not everyone agrees with the recommendation to drain the fuel at the end of the season as described in the section: General preventive maintenance.

    (From: Dwayne (

    There is some argument that draining all the fuel from the system is bad, allowing the carb to dry out and the inside of the fuel tank to rust. That was the case on my motorcycle; the guy who had it always drained the fuel for storage and it ruined the tank. I always just add fuel stabilizer to the tank, fill it completely, and run it for a short time and have never had problems.

    (From: Matt Howell (

    Fuel stabilizer's purpose is to prevent souring, and hence, the need to drain the fuel system before storage. In my experience, my equipment starts right up each season with stabilizer in the fuel. I would suggest you clean/rebuild the carburetor. Repair kits are cheap, and easy. Good luck!

    (Editor's comments).

    I have cleaned and rebuilt too many Tecumseh carburetors (mostly from neglected Craftsman lawn mowers). The cause in most of these was almost certainly gasoline left in the fuel tank between mowing seasons. You might get away with it for a couple of years but eventually the goop will prevail. I would definitely recommend draining the gas with these. The fuel tanks are plastic in any case and there are only a few steel parts in the carburetor and rusting of these is not that likely. A fuel stabilizer may not prevent the buildup of gunk and varnish as a result of the slow but inevitable process of fuel evaporation in the carburetor and replenishment from the fuel tank.

    For other types, I would still recommend draining the fuel tank and running the engine until the carburetor is dry. I believe that this will result in the best long term reliability in most cases. Now, if you live in a swamp and mow the seaweed.... :-)

    Testing a used lawn mower before you buy

    A used mower at a bargain price may not turn out to be such a bargain if you have to do extensive repairs. There are two types: the living and the dead.

    If the owner claims the mower will start and is prepared to demonstrate, this is usually a good sign! However, first, take a moment to check the following:

    (Disconnect the spark plug wire, and tie it safely away from the spark plug terminal to prevent accidental starting if you are doing anything more than looking.)

    If there is gasoline in the fuel tank and it will start without undo effort, then there is an excellent chance that the engine is in good condition.

    In most cases, if the engine starts reasonably easily, there will be no really serious problems. The ignition system may require a tune-up or the carburetor may need cleaning and/or adjustment. Even a hard-to-start mower may very likely restored to tip-top shape with this type of intermediate level maintenance. Of course, the blade may have to be sharpened or replaced.

    If the engine doesn't work - no gas in the fuel tank and no handy gas can is usually a tip off of this - how can you be fairly sure that there are no major mechanical problems? Note that the objective here is not to identify **the** problem but to have a good idea of whether repairs will be really expensive or difficult. Thus, we won't even bother checking the carburetor or spark as problems in these areas are minor compared to those caused by internal mechanical damage. Here are some simple tests you can do without tools and without overly upsetting the people running the sale or junk yard:

    WARNING: disconnect the spark plug wire and tie it safely away from the spark plug terminal if you will be doing anything under the deck. Yes, I know, there is nothing in the fuel tank but it doesn't hurt to be safe. Use a rag or proper work gloves if you attempt to rotate the blade directly.

    Assuming the engine doesn't flunk any of these tests, then you may end up with a functioning mower with relatively little additional cost and effort.

    Discussion of 2 stroke engine rebuilding

    "I am in the process of refurbishing an old Eska 7.5 H.P. outboard boat motor. The engine (a 2-stroke Tecumseh) runs fine when under load at full throttle.

    I can only get the engine to idle at a high RPM. If I lower the idle speed, the engine will eventually die.

    Once the engine is started and idling, any attempt to put it into gear will cause the engine to die. Sometimes it will stay running and I can get the throttle revved up at which point the motor will push the boat and run just fine. When I lower the engine speed (approaching the dock) the motor will eventually die.

    I am totally stumped. I've completely rebuilt the carburetor (new seals, etc.), replaced the condenser, points, and spark plug. I've set all adjustments to factory specs yet it still won't idle or go into gear."

    (From: Al Savage (

    Although I haven't worked in the field in 15 years, two stroke theory doesn't change much .

    Is your high idle (out of gear, unloaded) smooth? It shouldn't be. Two strokes without electronic mixture controls are almost always calibrated to be slightly rich when unloaded, as the nature of all three induction systems (piston port, reed, rotary valve) is such that adding a load leans the effective mixture. Something to do with flow dynamics, I didn't need to know, as I don't design them.

    You've covered the common problem area -- ignition. I suspect you have an air leak somewhere. Upper or lower crankshaft seal, reed plate gasket, intake gasket, and upper housing gasket are common areas, in that order.

    Mind, this is generic two cycle troubleshooting advice; I don't pretend to know the model you're working with.

    To track these things down in the shop, we'd build custom block off plates and pressurize the crankcase to a few inches, then watch the gauge to see the leakage rate. Not terribly practical unless you're rebuilding, but you sometimes found porous castings and cracks that way.

    When I'm feeling adventurous I'd spray starting fluid (with a tube nozzle attached) around all the seams with the engine running to see if the run behavior changed. Not recommended; too dangerous.

    Does your 2 stroke engine use reed valves?

    Since a bad reed valve can be the cause of a variety of problems, knowing if you even have these can be useful - not all 2 stroke engines use reed valves.

    (From: John Barry (

    One easy way to tell is to note where port is behind carburetor. If it just "dumps" into crankcase, like diametrically opposite cylinder, most likely it's got a reed-valve. If the passage from the carb connects to the cylinder at a point closer to the crank center-line than the exhaust port, perpendicular to cylinder axis and perpendicular to crank, it's mostlikely piston-ported. If the carb passage is parallel to the crank eventually (starts out perpendicular on Rotax engines), most likely you have a rotary-valve engine; this is usually the case for serious specific-output.

    Some problems with Briggs and Stratton engines

    (From: Mike Brandt (

    A quite common problem with these engines is the gasket between the carburetor and engine. It is usually a thick fiber gasket and sometimes gets blown out if engine ever backfires. This is more common with age, as the nuts holding carb on do sometime get loosened. Also gum buildup can and does clog the main and low-speed jets at times which means removing carburetor. Cleaning out with a good carburetor cleaner then blowing out all passages. This happens quite often here where they add 10% alcohol to gasoline. Gas goes bad a lot quicker than without alcohol!

    Comments on Briggs and Stratton repair

    The following was in response to a posting on about a 12 year old lawn mower. Of course, if properly maintained, such an engine may still have quite a bit of life left in it :-).

    (From: foxeye (

    The best method is if you do not have a flywheel puller is take it to the nearest Briggs dealer and have them remove it. Since you do have a puller, just put some tension on the flywheel with the puller you already have, and then hit the top of the puller (the threaded shaft that centers on the crankshaft end) with a heavy hammer (16 to 24 oz.) should do it. Sometimes all it takes is a sharp blow with tension exerted on the flywheel to break it free. One or 2 raps should be enough. If it fails at that, there are still other ways to remove it.

    Some have points and condenser some do not. There is a points/condenser replacement kit that makes it solid state available for less than $10.00 called Atomic Ignition. Also rust on the magnet of the flywheel and the ignition coil laminated portion can also cause it to have no spark. The measurement is critical for the ignition coil to flywheel space (called air gap) for proper ignition as well. Unless very badly corroded and worn, the points can usually be cleaned up and readjusted. Its not uncommon for the little fiber plunger that operates the points on some models to get stuck.

    You should pick up a manual, for proper dimensions, and bolt torque. Even though it may list special tools for some repairs, there is usually a work-around in most cases. I would not really spend to much money on this motor, as the age is against it, but a new one from a source like Northern Hydraulics can be had for $99 to maybe 125.00. I picked up a 5hp highwheel lawn mower with a Briggs I/C motor at the end of last summer at a local builders supply and garden center for $130.00 new in the box.

    Experience with Briggs & Stratton carburetor

    (From: Dan Hicks (

    Last Monday, while I was mowing the lawn, our 21-year-old Snapper mower quit on me. I could start it up but it would run for a few seconds and then quit again. After futzing with it in the yard for several minutes I took it back to the garage, and, after several more attempts at getting it going, I decided it was likely getting too much gas.

    I disassembled the carb and cleaned it good. It's a "PulsaJet" model with the carb mounted on the gas tank. The diaphragm looked good -- no visible holes.

    I reassembled things and the thing started and ran well enough to finish mowing, but it didn't have much "oomph".

    During the week I got a new diaphragm, and I got around to installing it today. What a difference!! It used to take several tries to start the mower, now it starts on one pull. And it has more power than it's had in years.

    Hard to say what may have been wrong with the diaphragm, but likely it had some microscopic holes in it.

    McCulloch chain saw hard to start

    "This weekend I picked up a 14" McCulloch chain saw and have had a hell of a time getting it started. I am no novice at this as I already have a gas trimmer and blower. I am sure my fuel mixture is right and have done just about everything else by the book. In how many pulls do I consistently start a cold McCulloch chain saw. I should mention that I HAVE started it but can't believe it should be this hard."

    (From: DanDee I had the same problem with mine!

    If yours is like mine , it has a throttle latch knob to use during starting. Mine never worked. After pulling myself silly trying to start it, I just decided to hold the throttle trigger open manually and it starts every time very easily.

    You may want to give it a try if you haven't tried it already.

    Riding mower stops periodically

    (From: Mike Odryna (

    You might want to check the oil level. Some of the Briggs & Stratton engines have a low oil pressure cut off. Some of these cutoffs are very sensitive. They will stop the engine when it's less than a cup low on oil. As far as I know, you can't adjust it either.

    How do you get the blade off a tractor mower?

    (From: Lloyd E. Sponenburgh (

    Typically, you remove the entire mowing deck, then manipulate the deck, rather than the whole mower.

    Alternatively, shops often LIFT the whole tractor, and work from underneath. I built a beam-supported block and tackle affair in my barn to do just that... it's lots faster than taking the deck off a stupid Murray. Those pieces of trash require that you remove about a dozen fittings and nuts to get a deck off.

    Now, my Wheel Horse, on the other hand, requires only ONE hand-released fastener, and off-she-comes!

    About Wico magnetos

    These are separate units (not part of the flywheel assembly) and may be found on larger (usually horizontal crank) engines.

    (From: Al Savage (

    The design is such that it uses an impulse spring to fire at low speed (less than 10 rpm, yes 10). It does a complex wind spring/release spring action until the engine fires. Other than the straightforward "replace the points/condenser" maintenance, I don't think I ever had to have the mechanical section apart, they're that reliable. I do think I had to clean up a couple of them, and relube them though.

    If you rotate the engine by hand (careful!) you should hear the impulse spring release, with a very audible bang. With the high tension lead hooked to your spare-plug-on-the-head, you should be able to have at least 3/8" spark; less is probably not enough.

    If you don't hear that loud bang from the mag, while very slowly cranking the engine by hand (two turns per bang on that impulse unit), you probably have a mechanical problem. If you hear the bang, but can't get the mag to fire repeatably, I'd refurb the points and condenser. It's a bit tricky, and I haven't even seen one in 15 years, much less worked on one in twenty, but there are many still in service and parts should be readily available.

    Oh, yes. I worked most of my teen years in small engine repair. So I *do* know whereof I speak. On that vintage equipment, anyway.

    What is inside a small engine electronic ignition module?

    The advantage of these systems is that there is no maintenance as there are no points to wear out or gum up. Timing is fixed by the relationship of the flywheel magnet and sensor. There are also no adjustments except to set the magneto coil pole piece to flywheel air gap.

    Replacement modules are available for many small engines to take the place of their original point type ignition system. These can be designed in several different ways but all must emulate the behavior of the point type ignition replacing just the magneto coil/pole piece assembly and without requiring alterations to the flywheel.

    Not that any of this profound knowledge does you any good for repair as these things are usually quite well potted and impregnable.

    Capacitor discharge ignitions systems

    (From: Matthias Meerwein (

    I once fixed a CDI unit for a 3-cylinder Evinrude 2-stroke outboard motor. The storage capacitor (1 uF, 400VDC) had a short. Quite simple to replace, but digging the PCB out of the silicone RTV sealing compound inside the ignition box was a real pain! But considering that Evinrude charges real big bucks (about 800 DM = 500$) for the ignition box, it was worth the effort. Another possible cause of failure of the CDI are the SCRs. Also, check the charging and trigger windings as well as the cables and connections.

    Which manufacturer makes what brand names?

    The following could be retitled: Sales people don't have a clue of what they are talking about most of the time or make something up to please the customer (and pad their commissions). I'm not sure how much of the following can be trusted though in all fairness, it may be that manufacturers switch around like musical chairs. :)

    (From: Tom Bellucco (

    I recently moved into a new house with a very large lawn and went shopping for a riding mower / lawn tractor. This is what I was told by various people, both in and out of mower sales:

    Now THAT'S confusion! I'd like to hear what anyone out there knows or has been told. It ought to be interesting to hear what other stores are telling people.

    Note: The guy that told me that MTD makes John Deere sells Toro and Ariens. He has publicly stated that he HATES John Deere (I won't post his exact words). I never realized how defensive a guy gets about his tractor -- you should have seen the reaction I got when I told a guy his John Deere was made by MTD! I probably got a worse reaction than if I said something bad about his wife!!!! Anyway, he got right on the horn to his John Deere dealer and was promptly told that John Deere makes all John Deere products.

    (From: Duane (

    Hmph... That *is* confusing. Just make sure that sucker says *Briggs & Stratton* on the blower cover!

    (From: John P. Curcio (

    According to my Cub Cadet dealer (from whom I purchased my snow blower), Cub Cadet, White, and MTD all come from the same factory. The only differences are color and features. They are located somewhere in Ohio, near Cleveland, if I recall correctly.

    (From: Carol (

    I live just up the road from the MTD plant - never saw a John Deere. I don't think they do that one.

    (From: Peter Szymonik (

    John Deere makes *all* of their riding mowers. The walk-behinds are assembled by a "third party", and many Deere dealers don't carry the walk behinds for that reason, and because the walk behinds had electrical problems.

    (From: BELJAN E (

    HUH??? That doesn't sound right at all. MTD makes that, MTD, Ariens, White, Cub Cadet, Yard Man, and Lawn Chief. Murray, another famous store brand makes some of them. Dynamark was and probably still is its own.

    John Deer is John Deer.

    (From: DeWayne McKay (

    You have several people misled on who makes what lawn equipment. I've been in the repair business for about 12 years now. Murry is made by Murry. John Deere is made by John Deere. There are several hundred names that can be found on the side of an MTD product, but MTD does not make any AYP, Noma, Dynamark, or Sears product. Sears in the late 80's was made by AYP and Murry depending on model. Now they are only made by AYP. The new line called Scotts is partially made by AYP. And, as for John Deere: The cheaper smaller tractors are AYP and the bigger tractors are made by John Deere. Huskvarna mowers are also made by AYP except a few high-end commercial models. Also the bottom-end or should I say cheap trimmers and chain saws from Huskvarna are made by Poulan. Have you ever heard of Gilson? They built a lot of the Sears products in the 70's and 80's and also built Lawnboy's tractor line and some of the Ford line in the late 80's.

    (Fr0om: Ernest Fields (

    Comment on modern manual mowers

    (From: Peter Szymonik (

    Sears still sells push mowers, I bought one this summer and use it for the trim areas, works great. The only downside is that you have to go to a professional mower shop to get the blades sharpened correctly.

    Comments on electric mowers

    (From: Leslie Gerstenfeld (

    I bought a Ryobi Mulchinator in '94 (cordless electric). I found it cheapest at Home Depot, I think it was about $325. I really like it.



    As for the big issue - how does it cut. Well, it cuts pretty well, but it certainly can't take down anything like a gas mower can. As the grass gets higher, it starts to leave a bit along one edge of the cutting strip slightly higher.

    If you are the type who knocks a foot off your grass every few weeks, this isn't the right mower. But if you mow regularly, it works pretty well.

    (From: Carol J J (

    I have used Black and Decker electric mowers for over 20 years. I Love them. We have a small property and a 100 foot extension cord is all we need. Since my husband is disabled, my young sons and I have done all the mowing for years. There are no worries about pouring or storing gasoline, no pulling like crazy to get it started. The blade is easy to replace. The grass bag is awkward but okay. My latest is a mulcher. It switches between regular and mulching easily. The mulch works well, not as good as a Snapper, but we're not paying Snapper prices either.

    (From: Nann Blaine Hilyard (

    I bought a Black & Decker cordless electric mower for $150 at Menard's last year. It is lightweight and very easy to use. I have not calculated the energy cost (we keep it quietly charging between mowings, though we unplug it in the winter) versus a gas mower, but I don't have the strength to pull the cord for a gas mower so the extra electricity doesn't bother me.

    (From: Geoffrey G. Shepherd (

    When my old Craftsman gas mower gave up this summer (gas leak) (what, you didn't read this FAQ?? :-) --- sam), I decided it was time to replace it with a new electric mower (my brother-in-law has the old mower now and is working on it for his own use). I ended up purchasing a Black & Decker CMM-1000 5 HP Cordless Mulching Mower from the local home improvement warehouse for just over $350.

    So far, I'm quite happy with it. It mulches better than the Craftsman did, and rear-bags when I want to add to the compost pile. It mows my entire city lot on a charge (in fact, the built-in meter still reads full charge when I'm done - admittedly, my house takes up a large portion of the lot). I also like how it adjusts height with one clever lever. It's fairly quiet, and my clothes don't smell like gasoline when I'm done. Call B&D at 1-800-762-6672 and they can send you literature and a free video on the CMM-1000.

    My only complaint with it is the weight. It seems to weigh about the same as the gas mower it replaced - it might even be few pounds heavier. But that's OK - I can use the exercise.

    (From: Steve Hill (

    I spent 3 years with a push reel mower. If the blades where sharp and the grass short then it was fine. If the blades where dull or out of adjustment or if the length of the grass to be cut at all long, it would leave the lawn a mess. I'd have to make 2 or 3 passes to get decent results. Also some types of weeds with resiliant stems where very difficult to cut.

    Finally, last year I bought a Black and Decker cordless electric. It was magnificent. It was a mulching mower so my raking days where over and it gave much better results. Also, it was very quiet and was always ready to go in an instant. All of my neighbors with gas mowers were very jealous.

    (From: William Lee (

    I have had a Ryobi for 4 years and found it to be adequate for our small lawns. It takes me about 20-30 do everything. In my opinion, it does not cut as well as a "good" gas powered mover that has a sharp blade. The batteries do degrade over time, although I'm am still working on the originals. It had an charger problem the first year, but that was fixed without charge. The blades are non-standard and I have an extra so that I can have a resharpened blade ready to go.I do enjoy not having to "listen" to the noise of a gas engine! Good luck!

    (From: Andy Dennie (

    As a former reel mower user and a current cordless electric mulching mower user, I can comment on both of these items.

    I used a reel mower for about a year and a half. I found that it worked well if I cut the grass pretty short, but when I did that I got more weeds. I tried raising the blade height, but then it didn't cut as well.

    This year I got a Ryobi Mulchinator cordless electric mower. So far I have been pretty happy with it. The charge is good for about 1-1.5 hours, supposedly, but it only takes me about 45 minutes, so I don't know what its limit is really. It's louder than the reel mower, but quieter than a gas mower. While charging it can stand on its nose and the handle can be folded up so that it takes very little floor space in my garage (this was important for me). One minor inconvenience is that you have to leave it plugged it all the time (at least that's what they recommend), so it is helpful if you have a plug near the place where you will store it (you don't want to be tripping across a long cord all the time). This didn't turn out to be a problem for me. Another thing is that you're not supposed to store it in very cold weather (bad for the battery I guess), so I had to move it inside recently for the winter.

    (From: Jim (

    The only problem we've had with our plastic-decked B&D is that it tends to collect grass when it's wet, and clogs frequently. We've chunked a few rocks with it, but with no damage.

    We haven't actually tried to cut rocks with it, and my experience is that a rock will penetrate the deck of any mower if it hits right. You should pick up the rocks before you mow, not try to hide from the shrapnel.

    In general, electric mowers are best suited for smaller lawns with less vigorous growth and without extensive landscaping to tangle the cord. A frequently sharpened blade makes a tremendous difference on electric mowers.

    (From: Robert Smits (

    I've had experience with both electric mowers and gas mowers. I live in an area where the grass can grow quite quickly and thickly, and the B&D electric one just doesn't have the oomph to cut long, thick grass. (Even when you make sure you're using as short a heavy duty cord as is practical - to avoid voltage drop in the cord). My 5HP gas model just whizzes through the same stuff, and not having to hassle with the cord is really an advantage - my lot size is just under an acre.

    The battery operated electrics may run out of steam quite soon if you have a larger lot, and count on replacing the battery pack every few years.

    (From: Roger Fillingim (

    I've had two electric mowers, both cordless - a Ryobi and a Black & Decker. The B&D was rated as a 5 HP engine, although I have no way of knowing whether it delivered the same power as a 5 HP gas mower. I used my electric mowers on a small lawn. I would agree that the electrics don't do as well on a overgrown lawn, but other than that my electric, especially the B&D, were great. I still have the B&D, but I'm in an apartment right now so I don't have much use for it. But, when our new house is completed, I plan to use it again.

    While charge time and power are potential downsides of electric, not having to change the oil, fill up with gas, pull a starter cord, etc... are substantial benefits in my opinion. Also, electric lawn mowers are much quieter and more environmentally friendly. If your yard is not too large and you mow fairly regularly, electrics are an option to consider.

    (From: Pat Kiewicz (

    I hung up my reel mower for my Black & Decker rechargeable electric, for the very same reason. My mower also can bag grass for those occasions when due to circumstances beyond my control I must cut grass that is too tall or too damp or when I want grass clips for mulch or composting.

    It runs on a lead-acid battery (same technology as your car battery) and can survive the winter in an unheated garage. I have to go rechargeable if I want to go electric as my lot stretches back 200 feet or more from the nearest plug. The battery should last a good, long time if not abused. (How often do you have to replace your car battery?)

    I believe the most recent issue of National Gardening Magazine has an article on electric mowers, with comparison between the features of different brands and models.

    (From: (C. Ligh).)

    I bought the Toro 24V rechargeable $349 based on the free video tape Toro provided describing all its features and options. The problem with the rechargeable is run time.

    I have a 10,000+ sq feet lawn and after about 35 minutes mowing the battery level lights went out. The owner's manual states: one should stop and recharge after the lights are out or the batteries' life will be shortened. Anyway there was still power left and I finished the lawn in about 65 minutes. With the prospect of replacing the batteries every year and run time shortened after every use, I returned the mower.

    Toro does offer a 30 day return. You might want to give it a try when you have grass. Get the corded model if you don't have too many trees and save a $100.

    When ever my 13 years old Honda dies, I'll probably get a corded electric mower. But I do miss the quietness of a electric and absence of exhaust fume.

    P.S. There is a 36V rechargeable, but it's a $100+ more. So, will replacing the two batteries = $200?

    (From: Michael Lamb (

    I own an electric mower now. It's a beefed-up Sunbeam. I found it on the side of the road and did a bit of 'customizing' to it. I like it MUCH better than the gas thing I had for years. The cord is a bit of a bother but when I think that I'm polluting far less and it uses far less power (costs about 25 cents less energy to cut the yard compared to the old gas one) AND it doesn't stink and be a pain to start and a health hazard and is quieter too. I might get a new cordless eventually but the $375 price tag is a bit much. I tried one out and think they are pretty nice, far lighter than a gas mower. Besides the one I have does very well for now. A friend of mine bought a B&D cordless, she likes it and has no problem doing her yard and her neighbors too! (she likes cutting grass)


    Have really enjoyed the convenience of a cordless mower. Bought it in 1992 and went to replace the battery. It is a model 3300m and got the shock off my life when the first place I tried quoted me a price of approx $236 cdn. I got in touch with B&D and they directed me to another dealer where I got it for $93. However, this dealer told me that the new price list does indeed show $236 cdn for new stock. Goodbye B&D the next time the battery needs replacing. I can buy a complete gas or ac unit for the amount of money.

    (From: Mike).

    The problem you state does not exist for only B&D lawn mowers, but *any* rechargeable product. Batteries do not last forever, and typically account for more than 50% of the cost in any tool. Often times the batteries are a standard size and you can get relatively inexpensive replacements at electronic stores. As an example, most cordless drills use sub 'C' cells which run about $1.75 a piece from Tanner Electronics in Dallas, Tx. For comparison, B&D wants $20 for their replacement (four batteries).

    At the risk of sounding like a zealot...the people that think they are "saving the environment" with their electric lawn mowers are fooling themselves. In the first place, all you've done is moved the pollution from your backyard to somebody elses, and soon we're going to extend this fallacy to cars (oh, joy...smog in the country). In the second place, the process used to produce Cadmium plating is so toxic that even the US military is phasing out its use, while most foreign (non-US) governments outlaw its production entirely. "Green factor" wise, we're all probably just as well off with oil burning tub-thumpers.

    I predict in the not so distant future (10-20 years) we'll see the end of NiCad powered appliances either because governments ban them, tax the daylights out of them, or nobody will be dumb enough to build the batteries anymore. At the very least, expect to see mandatory recycling programs for *all* types of batteries (not just lead-acid). Regardless, prices will sky rocket.

    Ok, soap box mode off :).

    (From: J. Matthew Good (

    First, battery powered lawn mowers use Lead - Sulfuric acid batteries, just like in your gas powered car, only smaller. Second, these batteries are one hundred percent recyclable. Yes, that is 100%.

    To the original poster: The only problem with battery mowers is that you must follow the directions about over- charging. Also, they are sealed, as most of them are installed laying on their sides, so ordinary replacement batteries would leak.

    Ryobi has replaced the pair of batteries in their machine with a single 24V unit which lists for about $95 US, PLUS FREIGHT. From what I've seen, they are lasting anywhere from 2 to 3 years for most users. To make it last longer, charge only until the indicator says it is fully charged, then unplug it. On a Ryobi, the light turns from red to green to indicate full charge, and supposedly goes into float charge. Don't take a chance, when it turns green, or whatever Black and Decker's equivalent is, Pull the Plug.

    (From: Ronald Kramer (

    This is my third year with a so-called 5HP Black and Decker battery electric. I think they are crap! I couldn't mow the lawn due to rain this weekend once again so the grass got to be twice the cutting height. This requires that I cut without the bag which causes the grass to clump. I then have to go over the area again with the bag. For my 12500 sq. ft. yard takes 4 days because I have only 50 min of charge. So when I finish I have a yard of grass at different heights and it is time to start mowing again.

    (From: Jeff Canavan (

    I recently had the decision to make regarding a new lawn mower. I went with all electric stuff for the yard; corded combo weedwacker/edger, 16" chainsaw and Black and Decker CMM1000 rechargeable lawn mower. Searching through prior usenet posts with, I found mostly good comments about the lawn mower and thought I'd give it a try. It comes with a 30 day money-back offer, so if I really hated it, I could return it to get a corded one or a fume belching fossil fueled model. After plugging it in overnight, I mowed the grass for the first time last Sunday, (it hadn't been mowed for 7+ weeks) After mulching the 1/2 acre of shin- to knee-high grass and weeds, the charge indicator was still in the green zone halfway between the Fullest and the yellow. The box states 1/2 acre range, including driveway, walkways, and house. It must be conservative or I got a really great set of batteries.

    Discounting all ecological debates regarding macro pollution issues;


    Pluses, neutrals, or minuses, (depending on your view of yardwork):


    (From: Frank Wilder (

    We have an 18" rechargeable Black & Decker electric and we really like it. We bought a refurbished unit at a Black & Decker outlet.



    I am happy with the one I have but I can't wait to upgrade to a bigger one. The newer B&D lawn mowers look really great.

    (From: Dave Clark (

    Country Home Products, the people who make the DR Field and brush mower offer a cordless lawn mower that has a removable battery. The battery has a built in handle and lifts out of the mower housing for easy replacement and charging. If it is half the quality of the DR Brush mower it is a great mower, and a great company to do business with. They are located in Vermont, and you can get their number from 800 information (1-800-555-1212).

    I don't own the mower (yet) but am considering it for my mothers house. I do own the DR and I love it. I have no interest in this company - but I am a very satisfied customer.

    (From: Topher Eliot (

    I'm happy with my Sears electric. I discovered on trick for dealing with the cord: use a heavy duty, long cord (100', 12-gauge in my case) with a short light cord at the end (20' 16 gauge). This allows moving back and forth a moderate amount without having to move that heavy cord.

    BTW, I don't think I really needed that 12-gauge; in retrospect a 14-gauge would have been better.

    Comments on mulching mowers

    (From: Jeff (

    Mulching mowers are nice, you don't have to bag any of the grass that you cut. Mulchers do need a bigger engine, that's why if they are used to mulch make sure it is at least a 5 hp 21" or less ( 6hp for a bigger cut). Mulching adds moisture and acts as fertilizer for the lawn. The bad thing is that you need to cut the lawn every four to five days. If the grass gets too long the mower can't cut the grass as well as it should (also it's not good for the grass if you cut off more than a inch at a time that is when the mower is set at 3 inches). So for that reason it would not look that good if it was to rain a lot and you could not cut the grass. As for me I have both types that works out very well.

    Comments on inexpensive lawn mowers

    True or false?:

    "Throw away mowers will always cost you more in the long run. Murrays and MTDs are bargain basement mowers, and you most definitely get what you pay for."

    "The problem with Murrays and MTDs is that they use the cheapest engines and components available. If you manage to get the engine to last, the rest of the mower will fall apart around it. Remember, the reason K-Mart, Lechmere, Caldor, and other mass merchandisers sell these machines, is because they want you to come back every 2-3 years to buy a new mower from them."

    "Murray is a bargain basement brand sold by mass merchandisers who want you to come back for a new mower every 2-3 years, and Tecumseh engines at about the same. :-( If you manage to get the engine to last, the deck will fall apart around it. On the bright side, at least they are cheap."

    Some responses:

    (From: Jan Hickman (

    Yea!! My bargain basement (cost under $100 when new) Murray is over 10 years old and I'm cutting .4 acres with it. You can bet when it wears out (if it ever does) I'm gonna buy some mega bucks mower!! But if you do take that bet, let me in on the other side of the action. Keep the blade sharp, the air filter clean, topped up on oil and they will run for a looooong time.

    (From: Don Sterner (

    Well, my Murray has a quality Briggs & Stratton engine which is still running fine. My problem is that the deck has rusted and is now a shell of its former self. There is barely enough metal left to hold the blade shafts in place. The mower is only 2 1/2 years old, but we do live on salt water (next to it, really). All of our property is well above the water line - the mower has never been wet with salt water. I've made it a practice to always hose the mower down after use and I keep it under cover when not in use. The only replacement decks I can locate cost nearly as much as a new mower.

    Our previous mower (a Craftsman) was returned to Sears within 6 months after it had lost much of its paint and was covered with rust.

    (From: BELJAN E (

    My MTD mower holds up fine, and I have the killer lawn. It burns out virtually every lawn mower's engine, whether it is Briggs or Tecumseh. I bought my MTD and it is a self propelled with 3.75 hp Briggs & Stratton Sprint engine. I would buy the same mower again, the problem is I probably wouldn't need to! The mower is so durable, it is unbelievable (I should note it is last years front cog drive which now is 4 hp quattro, and it is still sold with the 3.75 HP engine as Lawn Chief.) It survived a year and still doesn't burn oil! That is a major accomplishment. The only trick is to put Marvel Mystery Oil into the gas and oil. I also have a 16 year old Snapper and it still goes. Regardless, MTD is well worth it. You get a cheap mower that in my estimate unless you abuse it will last 5 years or more with good maintenance. $100 or so for 5 years of use (and it isn't hard to maintain them) is an excellent deal. A $600 mower could last less than that.

    (From: Floyd Reed (

    If you take care of amy mower, you can expect a reasonable amount of service out of it, usually comparable to the price. True, Murray mowers are cheaply built, but they build an awful lot of mowers... with different name brands. Most still carry a substantial warranty. As for Tecumseh engines, years ago they were the more expensive engine, placed on the more expensive machines. They were the only engine of this type and size to incorporate an actual oil pump. I never did care much for them as they liked to burn exhaust valves and blow head gaskets. The carburetors left a little to be desired also. They were good for my small engine repair business. I am out of the business now and have not taken one apart for a long time and do not know how they are being made today, but I'll bet if you take care of them and change the oil regularly as well as keep the air cleaner clean, you will receive respectable service out of it.

    (From: Jan Hickman (

    My Murray with a 3.5 HP Tecumseh engine is over 10 years old. The only parts replaced have been the wheels, the spark plug once and the blade a couple of times. The lawn area I cut is approximately 1/3 acre. Will I ever buy a $500 quality lawn mower? Not as long as I can get one to last like this. And by the way - it's not luck, it taking care of them. I know several people whose bargain basement brands last several years and they all have one thing in common. They take care of them."

    (From: Patrick J. McQuiggan (

    I've had a Murray since 1990. No problems whatsoever. Also 1/3 acre. Use it regularly. Follow maintenance advice. Engine is Briggs & Stratton 5 hp and it is self propelled. A bargain.

    (From: Bijan Mobasseri (

    I am amused with the high power recommendations here: Honda, Toro, Snapper, Lawnboy, some costing $600. In 1989 I bought a walk-behind Murray for $99. This morning, as in all Saturdays, I pulled it out of the shed, gave it ONE pull and as always it started. It is still running on the original plug (Tecumseh engine) and the only thing I have replaced is one $5 wheel.

    Comment on high wheeler and wish list

    (From: Edward Rice (

    I've got a non-propelled high-wheel, it's okay. Nothing very exciting. My lawn is hilly but not bumpy, and I had gotten the high-wheeler to assist in shifting over edging and over flag walkways, and it's okay for that - but not so great that I'd bother with a high-wheel design in the future.

    What I would *love* to get in the future, and maybe that 'weed whacker on steroids' is what I really need, is something that would let me go casually over edging, so the blade is cutting grass on one side and safely *not* cutting 1 to 2" river stones on the other side. This is made worse because the left-hand wheels on the stone (typically, with a right-throw mower) jiggle up and down enough that the occasional stone does get whupped.

    I'd also be interested, if anybody makes one, in an *offset* mower, in which the left-hand wheels were not to the left of the mower housing. By increasing the wheelbase and putting one fore and one aft, I would love for the left side of the mower to be cantilevered out with nothing under it but the blade, to let me get really close for edging. I don't want a specialized edger - I want a mower that can do the job through the simple expedient of getting the left-side wheels out of the way!

    Comments on plastic decks

    (From: Joe Kowalski (

    I bought a new Honda with a plastic deck. I like it. The plastic is not like regular plastic; in fact, it has proven bulletproof. It is very lightweight. I mowed many rocks, keys, etc. with no problem. It was a concern of mine when I bought the unit ($660), but the salesman convinced me that it was 'put through hell' to prove its vitality.

    Comments on Briggs & Stratton versus Tecumseh engines

    These two manufacturers produce the vast majority of small engines used in low to medium priced yard equipment.

    I have always preferred Tecumseh engines over Briggs & Stratton though perhaps this is more of a touchy-feely thing than representing any really fundamental difference between the two. The float carburetors (e.g., Craftsman) do tend to gum up if the gas is not drained at the end of the season but they are easy to clean and rebuild. (Also, I have picked up several mowers dirt cheap at garage sales where the only problem was a gummed up carburetor - 15 minutes work to remedy.) Change the oil regularly and Tecumseh engines will keep going for a long time.

    (From: BELJAN E (

    What a JOKE!!! Try rebuilding a '72 carb sometimes, the floats are terrible, I finally slapped a '91 4 HP carb on the 4 HP '72 and it works now!!!! Some newer ones can be saved, but 10 years of sitting with *leaded* gas, that spells the end.

    (From: Sam.)

    Hehehe... A guy just sent me a couple of carbs that were probably closer to '72 than '91. A new needle, seat, and hinge pin (in one case) was all that was needed (beyond the gallon of carburetor cleaner!). :-)

    (From: BELJAN E (

    It depends on how the gas was in, if you drain the float, you should be able to get it clean. Another trick is to put Marvel Mystery Oil in the float assembly and let it sit there a week, in 99% of cases, that will get it running. On the '72 carb, The gas formed crystals around all of the parts, I put all sorts of carb cleaners in, and I replaced the rubber seal, float, needle, and all replaceable parts, it didn't work. I pulled the carb off (this happens to be a '72 Tecumseh off a Sears engine. My guess is that is was overhauled so I got it starting on one pull with starting fluid). I took a '91 Sears (Tecumseh) carb off and bolted it onto the engine, since this is a power reel mower, the engine isn't easily replaced, the new carb is from a primer type non-choke engine, so I made a rubber plug in the hose to the air filter assembly to spray starting fluid into the carb to start it. Now it works fine. It looks strange but runs great!

    (From: Thomas N. Harding (

    I would like to add that Tecumseh engines are easy to rebuild. Most folks would be surprised how few parts are in them.

    (From: Steve Ordinetz (

    Several years ago I worked with a guy who rebuilt small engines as a sideline, and he was partial to Briggs & Stratton because parts tended to be pretty standard from year to year, while Tecumseh were less interchangeable between similar engines of different vintage.

    (From: Terry Highley (

    I agree 100%!!!

    (From: BELJAN E (

    Ha! I have had no trouble forcing Tecumseh engine parts to fit on same horsepower engines, and they all worked fine, I had spent over 30 minutes starting one with stale gas once though!

    (From: jakdedert (

    IME, Briggs motors are much more 'finicky' for starting and maintenance, but they last longer and use much less gas and oil.

    The Tecumseh's (I have two of those, and one Briggs & Stratton) start much easier, although that's probably due to the priming bulb which my Briggs does not have. They start using oil right out of the box, however. I have to check and add oil on almost every use. My Eager1 uses almost twice as much fuel to mow the same yard as my older Briggs of the same horsepower and cut width. I almost never have to add oil to the Briggs in an entire season.

    Given the greater oil and gas consumption, I assume that the Tecumseh's are emitting considerably more pollutants per hour of use. The consumables only add a negligible amount to my lawn-care costs every year; but multiplied by (however many millions) of them out there.....

    Any thoughts?

    (From: James Sweet (

    Well it's hard to say these days, back when I was tinkering with this stuff a lot I was dealing with engines made mostly in the 1970's and the Briggs motors were very noticeably better made and easier to work on. The Tecumsehs' were nothing but a pain in the ass, carb problems, ignition problems, a few common models would regularly throw rods. It was no shock since they were always about 40% cheaper than a comparable B&S. Now days I don't really know, the Tecumseh engines seem to have improved somewhat, while the Briggs engines have gotten *very* noticeably poorer. Somebody said they're now made in Asia so perhaps that's when the quality took a dive. BTW the new ones do have a primer bulb, haven't tried starting one though. My mom has an old mower with a Tecumseh and it does actually start quite easily though I have to take apart the carb and clean out the banjo bolt on the float bowl every year, never had to do that with an old Briggs. Unfortunately I never compared fuel consumption but I don't recall oil usage problems on either.


    Yes, there was a time when Briggs were better engines. Not so anymore. B&S has gotten into the parts game. They make nearly as much profit on parts and service as they do in the initial sale. Tecumseh had to rethink their whole market strategy in the late 70's and refocused on building better quality engines that were certainly more standardized. On virtually every Tecumseh vertical shaft engine the carbs and coils are interchangeable, unlike Briggs where every engine requires a particular model coil or carburetor-setup.

    Odd that your Eager-1 (Sears) mower uses so much oil. I'd say it might be just the particular engine as virtually none of the ones I've used or serviced ever developed this problem during their useful life.(Cast iron sleeves and 2 oil rings unlike the single ring in Briggs and the cheap alloys used in both the sleeve and ring.) The poorer, cheap design in the Briggs carbs make for a much worse problem in regards to pollution as they tend to run rich with just a little age.(I've seen some come into my shop that are running so rich that they actually wash the cylinder. This results in excessive wear on the sleeve and rings, allowing bypass into the crankcase. Once the oil is broken down by the gas it'll bypass the rings and the result is a smoking engine.(great if you want to fog your yard as you cut it.) Tecumseh carbs are actually easy to operate and care for once you learn some basics.

    They're float style carbs and as such you can't just run 'em and forget 'em. leaving gas in the carb and tank off season is the worst thing you can do. (Most people don't realize that gasoline actually goes bad with time and actually expect a mower that's sat unused for six months to just crank up on the first pull.) Run the tank dry at the last cutting of the season then loosen the Bowl nut and drain the gas out of the float bowl.(It's easy, all it takes is a 1/2 inch socket to loosen it.) Then leave it dry for the winter, DON'T put any type of winterizer in it. Worst stuff in the world. Just ensures you'll be taking it in for a carb job as the stuff just turns to jelly and will make a real mess of your carb and tank. There's just no way to "preserve" gas, use it or lose it.

    If you want proof as to Tecumseh's quality over Briggs just look at how Briggs has copied Tecumseh design. Tecumseh has the best design in a starter recoil mechanism hands down, Briggs finally gave up using their ball bearing starter clutch mechanism on all their 5 hp or larger engines and copied the Tecumseh design out right. (But it's still not as good.) They went to a float style carb but picked what is in my opinion one of the worst makes, Walbro. I tell all my customers if they insist on a Briggs, just get one of the Sprints, either a 3.5 or 4 hp model, and don't expect much more than 2 years of use out of it. I have some customers still using the same Craftsmans mower with a Tecumseh engine for 10 years or longer. I gave my in-laws one of my personal Craftsmans that I purchased back in 1982 and it's still going strong, with NO major engine work at all.

    (From: Sam.)

    My $35 garage sale Craftsman mower with the basic 3.5 hp Tecumseh engine is still going strong after 24+ years. The ignition module went bad a couple years ago (replaced from junk parts I had), and the carb needed cleaning once. I do usually try to remember to drain the carb after the mowing season but it doesn't always happen. :) Aside from those problems, it still usually starts on 1 pull.

    Comments on trimmers

    (From: Alan Dale (

    I have used several Ryobi 15" grass trimmers over the years and I've learned a few things in the process. Every now and then, they just simply will not crank. A good spark plug cleaning will remedy that.

    Also, apparently their 31 cc engine is not big enough, or other aspects of the machines are not heavy duty enugh to handle those 3 metal bladed cutters some places offer as "after market" wonders. I have used two of them over the years and both times it was disastrous to the trimmers.

    The heavier 0.105" 4 sided red cutting line does a great job on these trimmers. It cuts much better than the 0.080" round line and lasts much much longer. You have to open up the holes in the cutter head to make the bigger line fit but that is no problem. Unless, of course you decide to go back to the smaller line later.

    Comments on Honda overhaul

    (From: Israel Kantorowicz (

    I recently damaged the engine on my Honda mower, so here is my experience (Seattle area):

    The engine is well built and fairly easy to work on. Their manual specifies a bunch of special tools, but I was able to take the engine apart and put it back together with just "ordinary" small engine tools. The only problem I had was with the flywheel. It has no threaded holes, and needs to be held around the periphery with something like a belt-wrench while torquing the center bolt. For the same reason, a large gear-puller is required to remove the flywheel from the crankshaft. I did not attempt anything ambitious like replacing valve seats, though. All threads on this engine are metric, of course.

    The manual (available from dealers) is very clear and well illustrated. Carb rebuilding is not covered, however. Part numbers are listed in a special "parts manual", not in the regular one.

    Note that Honda appears to have a policy of not letting anybody but their dealers get in touch with their central tech support. They don't publish their phone number, and there is no email address either. I could not find the timing mark on the crank, and the dealer could not or would not help, so guessing was the only option left... I don't know if other small engine makers are any better in this respect, but I am used to customer support meaning something entirely different in my line of work.

    No trouble getting parts, but they all had to be "special ordered" from out of state, taking about a week to arrive. They seem very expensive to me. A rod was $50+, a set of piston rings, valve springs, or head gasket $20+ each, and the crank (which I luckily ended up not needing) was supposed to set me back about $185. My relatively minor repair ended up being about $125 in parts (taxes, oil, and band-aid for bruised knuckles not included).

    The mower has served me well, and the only problem other than caused by my own fault was premature wear of the blade clutch lining. The plastic deck seems to take rock hits much better than an aluminum deck that I owned some time ago, and there is no paint to peel of from it.

    Walk behind versus riding mower or tractor

    (From: Stephen M. Henning (

    The forward speed is about the same for a walk behind and a rider, about 3 mph. The main variable is the width of the mower. Let's say we have a 42" mower. If the overlap is 4", then we have 38" cut.

    Then the time to do one acre with a 42" mower is:

    220 ft. x 220 ft. / ( 3.17 ft x 4.4 ft/sec x 60 sec/min) = 58 minutes.

    In summary:

    You can go faster with a rider, but the quality of cut degrades. The walker gets slower for a large lawn since one gets tired and takes more breaks and tends to slow down on the turns.

    I would recommend only a tractor that had a cast iron engine and a rear discharge mower. I had a tractor with an engine with aluminum heads. The first engine failed and I replaced it and the second failed. They both warped and eventually failed. I got a tractor with a cast iron engine and it never quit. Eventually it got so old that when the fuel pump failed, it could not be replaced. Now I am using a WheelHorse/Toro with a 17 HP Kohler 2-cylinder cast iron engine. It is a great machine and has worked flawlessly over 12 seasons. The 17 HP is excessive for mowing or pushing snow, but the 2-cylinder engine is great. I can mow for 5 hours and not get the least bit fatigued. However, with 1-cylinder engines the vibration was so great that my toes and fingers would become numb in less than an hour. I had to take a break every hour to recover.

    For the mower, get a steel rear discharge mower. The first mower I got was a cast aluminum mower. Every rock the mower picked up knocked a chunk out of the housing until there was nothing left. Then I got a steel mower, but the aluminum engine gave out and it was not prudent to get a third engine for the steel mower. I did get a tractor with a cast iron engine that could use the steel mower and this combination lasted 20 years.

    Now on the WheelHorse/Toro I got a rear discharge mower because it does not throw objects like the side discharge machines. Also, when the grass clippings are too heavy to leave on the ground, I can collect them with a simple sweeper and not need a motorized collection system. Also, it doesn't throw clippings on the flower beds and walks or make passing motorists think they are going to be attacked. It is also easy to mow next to shrubs and trees on either side of the tractor. You don't have to always mow with the discharge to the mowed or unmowed side, depending upon what you are trying to accomplish.

    A lawn mower for the security minded

    Smarter (and lower maintenance) than the average cow...

    "This is a lawn mower that is solar powered and stays within a buried wire boundary. It also 'learns' the layout of your yard and will even put itself away at night. Does anyone have one of these devices? It sounds great, but at $1,500, I would like some real world product feedback."


    I don't have one, but apparently the CIA has two. They use them in a courtyard that is completely surrounded by one of their buildings. The robo-mowers were seen as a better solution than having to have a security officer escort some person in/out and watch them mow.

    The miracles of JB Weld

    OK, I have no idea of whether the following repair will actually hold up but I supposed there is nothing to lose by trying:

    (From: Andrew Bowers (

    I just did something cool!

    My friend Dan gave me a 3.5 HP Briggs & Stratton horizontal shaft engine, which burned oil like crazy. I took it completely apart, (Dan had started to, but he hadn't taken the actual block apart, he had just taken the carburetor, muffler, and flywheel cover off) and found a _deep_ gouge in the cylinder wall. The rings were fine, except there was a chunk missing from the oil ring. Here comes the "miracle" part. Most people would have just chucked the engine, or kept it around for parts. I grabbed my thing of JB Weld ("the world's finest cold weld"), mixed some up, and filled in the gouge. The next day, I sanded the patch smooth, and put the engine back together. Started on the first try. Still burns oil (due to the missing segment on the oil ring; the auto parts store I went to didn't have piston rings, can you believe that?), but runs pretty good. (Well, the injectors in the carburetor keep getting clogged, and it won't start, so we're going to get a new, *plastic* gas tank).

    Interesting lawn mower repairs

    (From: Sean Smith (

    When I was 17 and still living at home, mowing the lawn was part of my chores. Well, I didn't have a running lawn mower and my Mom insisted that I mow the lawn. She pretty much stated that she didn't care how I did it, as long as it got done.

    So, I figured that I needed to get one of them running, and I couldn't for the life of me figure out what the exact problem was but I wasn't getting spark, so I figured that it had to be the magneto. Well, I didn't have the resources to get a new one, so I did the next best thing, I rewired it!

    Mom comes out to find me mowing the lawn with a car battery and a coil strapped to the thing. Ran a line from the condenser to a spare coil I had for my VW Bug and strapped an AC/Delco to the thing and away I went. It was a bit heavy but it was better than nothing. I still wish I took pictures of it.

    (From: Charles Gilley (

    My coolest repair was rebuilding my B&S after letting the oil run out. After I put it back together and with great anticipation, I began to pull and pull and pull.... I was impressed. It was silky smooth, but the darn thing would not start. I sat down to rest, glanced at the bench... to see the valve lifters still sitting there.


    Importance of having the correct valve clearance

    Since would be hard to have valve-clearances of all engines here, look in the owner's manual. Many manufacturers have them on-line. See the section: Links to engine and equipment manufacturers. A search engine (no pun...) will also find these quickly. (From: Derrick Setchell (

    My father-in-law owned a Flymo with a 3.5 HP Briggs & Stratton. He'd had it for nearly twenty years, but never used it much because it was "hard to start", especially when hot. He has other mowers and neither of us had found time to investigate it. Why he didn't return it for warranty investigation at the start is lost in the mists of time.

    This year, he felt it was taking up space in his barn and was going to throw it away, so it was rescued by yours truly. I thought the motor might be a useful standby in case the even older Aspera (European Tecumseh) on my own rotary packed up. That's been a fine engine, by the way. The B&S was in a pretty good state considering, but sure enough took up to fifteen pulls to start when cold and would not re-start at all if fully warmed up. It did look like it'd been HOT, with the usual blockage of ventilation spaces by clippings, but otherwise well preserved.

    I gave it a complete carb and magneto overhaul which improved cold starting somewhat, but not the hot. I also felt it was a bit "gutless", so then investigated the combustion chamber and valves. This poor machine had virtually NO valve clearance when cold, on either lifter. The valves were still in pretty good order, so my assumption is that it was shipped that way from the factory. Once it got hot, the compression must have been zero, though it didn't obviously seem to be.

    A thorough clean, careful adjustment of the valve stems with a diamond hone (out of the engine !! ) to provide the upper limit of the specified cold clearances and back together it went. What a contrast ! It now starts first pull, hot or cold, and runs like it should have done when it left the factory. My guess is the magneto and carb were fine all along in this lightly-used engine. Next time I'm going to look at those valve clearances earlier in the sequence, even if the engine isn't old....

    Valve adjustment for OHV engines

    (From: Mike's Small Engine (

    You don't adjust OHV engines at TDC. You must bypass the compression release by turning passed TDC and letting the piston drop 1/4". Using a plastic ball point pen as a tool in your spark plug hole you can let the piston push the ball point pen out of the spark plug hole till it starts returning inside the cylinder. Make a mark on the pen so you can see the 1/4" drop of the pen. Then set your valves at 0.004 clearance for both IN/EX. Note: This is a general valve clearance specification that covers most engines today. Check your maintenance book or ask your local dealer for the info on the valve clearance for your specific engine.

  • Back to Small Engine Repair FAQ Table of Contents.

    Internal Combustion Engines

    Small engine technology

    If you have some idea of how your automobile engine operates - or a Model T Ford for that matter - then you know the basic operating principles of your small engine as well. In fact, your Craftsman Eager 1 has a lot more in common with a Model T than a Honda Accord. However, strip off, the electronics, pollution control devices, and engine powered accessories, and the basic mechanical construction is very similar, though the lawn mower engine is not manufactured to quite the same tolerances and with the same quality materials as an automobile engine.

    Nearly all small engines up to 20 HP or so are single cylinder affairs - one piston, one spark plug, no distributor, forced air cooled - about as simple and straightforward as it gets.

    If you have never been under the hood of your automobile, then the description in the following sections may be of some help.

    The next chapter: "Engine Diagrams" provides an explanation of each of the 4 strokes of a 4 stroke engine. However, if you cannot get the hang of my fabulous ASCII graphics, check out the following site:

    The How Stuff Works Web site has some really nice introductory material (with graphics) on a variety of topics relating to technology in the modern world. Of relevance to this document is an article on "How Car Engines Work" which is really mostly about the basic principles but WITH some real animated graphics!

    Types of engines

    Unless otherwise noted, most of the descriptions and procedures in this document apply to both 4 stroke and 2 stroke engines. However, there are fundamental differences in the proper fuel and oil that is used with each type.

    The small 4 stroke engine has a separate oil sump just like the engine in an automobile. Therefore, gasoline and oil are separate. Oil changes are also required.

    WARNING: a new lawn mower or other piece of yard equipment will very likely be shipped without oil or just a minimal oil fill. Check it first and add oil if necessary. Running an engine without oil for a few minutes can cause serious - or terminal - damage. Even if your mower was assembled by the store where you bought it, don't assume they filled it with oil and tried it out!

    The 2 stroke engine requires that the gas and special oil be mixed prior to use in specific proportions. Leave out the oil - or get your gas cans mixed up - and you will quickly ruin a 2 stroke engine due to lack of lubrication if plain gas is used by mistake. Clearly label the gas cans for each type and instruct anyone using them in the proper fueling technique.

    Portable tools like chain saws, weed whackers, and backpack type leaf blowers use 2 stroke engines as these need to operate in a variety of positions.

    Stationary or wheel-about equipment including most lawn mowers, rototillers, shredders, backup electric generators, and large blower/vacs, use 4 stroke engines.

    Another distinction is that engines smaller than about 2 horsepower are generally 2 stroke while those larger than 2 horsepower are generally 4 stroke but there are exceptions. Lawnboy lawn mowers tend to have 2 stroke engines and there are some types of equipment with very small 4 stroke engines. Of course, if your engine has a cap marked 'oil' then it is a 4 stroke.

    Larger pieces of yard equipment like riding mowers and lawn tractors use 4 stroke engines that are really very similar in most respects to their smaller cousins - much more so than to the engine in your automobile, for example. Similar servicing procedures apply. In fact, if you read the respective chapters in any of the engine repair books for engines (listed in the section: References) under 5 horsepower and those between 5 and about 20 horsepower, the only significant differences will be in the size of the various engine parts!

    Parts of a 4 stroke engine

    You may be surprised at the large number of individual parts which comprise the engine even on a $100 mower. The following description is for a typical single cylinder 4 stroke engine as would be found on most rotary mowers, rototillers, shredders, backup electric generators, larger snow throwers and leaf blowers, and even modest size riding mowers and lawn tractors:

    Engine operating principles

    These are internal combustion engines which means that the burning of the fuel-air mixture itself powers the engine. External combustion engines use the heat from combustion to expand or boil a working fluid as in a steam engine. Other examples of internal combustion engines are the rotary Wankel engine and gas turbines (jet engines).

    The type of engine in your lawn mower or automobile operates on what is called the 'Otto' cycle (if you care). A complete 'cycle' is needed to supply one power impulse to the output shaft. All engines must provide the following regardless of whether they are 2 stroke or 4 stroke, rotary, or turbines (though turbines or jet engines operate in a continuous rather than pulsed manner):

    For the following, refer to the section: The four strokes of a four stroke engine in living ASCII art.

    1. Intake stroke. Air is mixed with fuel (gasoline for piston engines) and drawn into the combustion engine.

      4 stroke: The air-fuel mixture is sucked into the cylinder through the open intake valve as the piston moves downward on the intake stroke.

    2. Compression stroke. The air-fuel mixture is squeezed into a smaller space. This heats it to some extent and prepares it to be burnt. (Note: in a diesel engine, this heating alone causes the mixture to ignite and there is no spark needed). Compression ratios for small engines are typically low compared to automobile engines.

      4 stroke: Both valves are closed. The piston moves upwards thus reducing the space above it and compressing the air-fuel mixture.

    3. Power stroke. The compressed air-fuel mixture is ignited at a precise time by the ignition system (spark).

      4 stroke: Both valves are closed. The heat produced by the rapidly burning gases to expand and drive the piston downward and because it is connected to the crankshaft, drives the load as well.

    4. Exhaust exhaust. The burnt combustion products are driven out of the cylinder. These consist of carbon dioxide, carbon monoxide, water vapor, oxides of nitrogen, some unburnt hydrocarbons, and numerous other mostly harmful compounds.

      4 stroke: The exhaust valve is open. The piston moves upward and pushes the exhaust gasses out through the muffler.

    A relatively massive flywheel attached to the crankshaft provides the inertia to allow the engine to coast through the non-power strokes (1-3). However, this is not always enough by itself - the blade on a rotary lawn mower is often required as well and a rotary mower may not start easily if at all without the blade in place and tightened securely.

    Note that the terms '2 cycle' or '4 cycle' are often used incorrectly when what is meant is 2 stroke or 4 stroke. The cycle is the entire sequence of events including intake, compression, power, and exhaust. The complete cycle for a 4 stroke engine is two complete revolutions of the crankshaft. The complete cycle for a 2 stroke engine is one rotation of the crankshaft. This means that a 2 stroke engine produces a power stroke on every rotation of the crankshaft while a 4 stroke engine does this only on every other one. Thus, a 2 stroke engine will be more powerful than a similar size 4 stroke engine. However, on the down side, 2 stroke engines tend to be less efficient in fuel utilization and pollute much more than 4 stroke engines.

    Bearings and bushings

    The shafts of rotating parts normally are mounted in such a way that friction is minimized - to the extent needed for the application. A bearing is any such joint with more specific terms used to describe the typical types found in lawn mowers - or small motors, automobile engines, or 100 MW turbines.

    The bearings to be concerned with in a lawn mower or small engine are:

    A variety of bearing types are available. For most inexpensive rotary lawn mowers, plain bearings are most popular due to their simplicity and low cost.

    Crankshaft anatomy

    The diagram below shows the major parts of the crankshaft:

                               Counterbalance weights
                                    __   |   __
                                   |  |  |  |  |
           Threads for             |  |<-+->|  |           Threads for
           Flywheel nut            |  |     |  |           blade adapter nut
                 |                 |  |     |  |  __              |
                 V     _________---|  |     |  |-|__|_______      V
                ...---/         |  |  |     |  | |__|       ----+...
                |||   |         |  |  |     |  | |__|       |   ||||
                '''---\_________|  |  |     |  | |__|_______----+'''  
                             ^  ---|  |_____|  |-|__|  ^          
                             |     |--|     |--|  ^    |
                    Main bearing   |  |     |  |  |   Main bearing
                   (Flywheel end)  |  |_____|  |  |  (PTO/blade end)
                                   |__|  ^  |__|  |
                                         |        +--- Crank gear (meshes
                                                       with cam gear)
                                     Crank pin

    Basic operation of a float type carburetor

    The diagram below shows a schematic of a typical float type carburetor with the engine running at high speed. The choke plate is fully open and the throttle plate is opened the proper amount be feedback from the governor to maintain the speed set by the user throttle control.

                Air filter       (Air: -->, Air+fuel: ==>)
             /             \  Choke plate               Throttle plate
              | ||||||||| |  (Fully open) Venturi      (Partially open)  
              | ||||||||| |______________    v   _______________________________
              | |||||||||                \______/
              | ||||||||| -> -> -> -> ->           ==> ==>      /  ==>  Intake
              | ||||||||| ->  ----O----  ---> ===> ==> ==>    O    ==>  pipe to 
              |           -> -> -> -> ->  __||__   ==> ==>  / |    ==>  cylinder
               \____________________  ___/  ||  \_____________|__________________
                                    ||      ||                |
        Fuel __________ Air bleed ->||      ||<- Main nozzle  o---------+ 
       Inlet ______    |____________||______||__                        |--> Pull
                |  |_ _|                    ||  \       Speed <--/\/\/--+    from
       Needle ->|  __A_______________       ||   |     control   Spring    governor
       Valve    | |o__/              \      ||   |                          closes
                |-----|     Float     |-----||---|                         throttle
                | . . |               | Gas || . |<- Float bowl              plate
                | . . .\_____________/. . . || . |

    The four strokes of a four stroke engine in living ASCII art

    You will just love the most excellent, accurate, and to-scale (:-).)ASCII renditions below but at least it will display on any computer using a fixed width font and not require half your disk space allocation for storage!

    Note that the valves, normal operated by a camshaft driven off of a small gear on the crankshaft, are not drawn in their usual position for a common small (side valve) engine to improve the superb clarity of these drawings.

    Intake stroke (1)

    Air-fuel mixture is sucked into the cylinder by the by piston moving down driven by inertia of flywheel (and blade).

        From ignition/magneto coil >========
                               Spark Plug ||||
                _________________________| || |________________________
               |  _______________________| || |______________________  |
               | |                          =="                      | |
               | |   ______   ->  ->  ->  ->  ->  ->  ->  ->  ->     | |
               | |__ \____/ ___   ->  ->  ->  ->  ->    ___ ______ __| |
               |____\  ||  /   | _____________________ |   \\____//____|
    Air-fuel           ||  |   ||                     ||   |  ||
    mixture from --->  ||  |   ||=====================||   |  ||
    carburetor  ______ || /    ||=====================||    \ || ______ 
               |  ____||||___  ||=====================||  ___||||____  |
               | |     ||    | ||        |            || |    ||     | |
               | |     ||    | || Piston | O          || |    ||     | |
               | |           | ||        V  \         || |    ||     | |
               | |           | ||  _ _ _ _ _ \ _ _ _  || |           | |
               | |   Intake  | ||/           \       \|| |  Exhaust  | |
               | |   Valve   | |              \        | |  Valve    | |
               | |   Open    | |              \        | |  Closed   | |
               | |           | |          Rod  \       | |           | |
               | |           | |               \       | |           | |
                                            ---> O  Crank pin journal
                                     Main bearings

    Compression stroke (2)

    Air-fuel mixture is compressed as piston rises in sealed volume of combustion chamber driven by inertia of flywheel (and blade).

        From ignition/magneto coil >========
                              Spark Plug  ||||
                _________________________| || |________________________
               |  _______________________| || |______________________  |
               | |                          =="                      | |
               | |               ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^               | |
               | |__ ______ ___  | | | | | | | | | | |  ___ ______ __| |
               |____\\____//   | _____________________ |   \\____//____|
                       ||  |   ||                     ||   |  ||
                       ||  |   ||=====================||   |  ||
                ______ || /    ||=====================||    \ || ______ 
               |  ____||||___  ||=====================||  ___||||____  |
               | |     ||    | ||            ^        || |    ||     | |
               | |     ||    | ||  Piston  O |        || |    ||     | |
               | |     ||    | ||         /  |        || |    ||     | |
               | |           | ||  _ _ _ / _ _ _ _ _  || |           | |
               | |   Intake  | ||/       /           \|| |  Exhaust  | |
               | |   Valve   | |        /              | |  Valve    | |
               | |   Closed  | |        /              | |  Closed   | |
               | |           | |       /  Rod          | |           | |
               | |           | |       /               | |           | |
                  Crank pin journal  O    
                                     ^  \ 
                                     |    \ 
                                     Main bearings


    Compressed air-fuel mixture is ignited just before piston passes Top Dead Center (TDC) and starts down again.

        From ignition/magneto coil >========
                              Spark Plug  ||||
                _________________________| || |________________________
               |  _______________________| || |______________________  |
               | |                          =="                      | |
               | |                       - X -                       | |
               | |__ ______ ___  _____________________  ___ ______ __| |
               |____\\____//   ||                     ||   \\____//____|
                       ||  |   ||=====================||   |  ||
                       ||  |   ||=====================||   |  ||
                ______ || /    ||=====================||    \ || ______ 
               |  ____||||___  ||                     ||  ___||||____  |
               | |     ||    | || Piston   O          || |    ||     | |
               | |     ||    | ||          |          || |    ||     | |
               | |     ||    | ||  _ _ _ _ | _ _ _ _  || |    ||     | |
               | |           | ||/         |         \|| |           | |
               | |   Intake  | |           |           | |  Exhaust  | |
               | |   Valve   | |      Rod  |           | |  Valve    | |
               | |   Closed  | |           |           | |  Closed   | |
               | |           | |           |           | |           | |
                                      ---> O
                                           | Crank pin journal
                                     Main bearings

    Power stroke (3)

    Heated expanding gases drive piston downward and apply power impulse to crankshaft via connecting rod (clockwise rotation).

        From ignition/magneto coil >========
                              Spark Plug  ||||
                _________________________| || |________________________
               |  _______________________| || |______________________  |
               | |                          =="                      | |
               | |               | | | | | | | | | | |               | |
               | |__ ______ ___  V V V V V V V V V V V  ___ ______ __| |
               |____\\____//   | _____________________ |   \\____//____|
                       ||  |   ||                     ||   |  ||
                       ||  |   ||=====================||   |  ||
                ______ || /    ||=====================||    \ || ______ 
               |  ____||||___  ||=====================||  ___||||____  |
               | |     ||    | ||        |            || |    ||     | |
               | |     ||    | || Piston | O          || |    ||     | |
               | |     ||    | ||        V  \         || |    ||     | |
               | |           | ||  _ _ _ _ _ \ _ _ _  || |           | |
               | |   Intake  | ||/           \       \|| |  Exhaust  | |
               | |   Valve   | |              \        | |  Valve    | |
               | |   Closed  | |              \        | |  Closed   | |
               | |           | |          Rod  \       | |           | |
               | |           | |               \       | |           | |
                                                 O  Crank pin journal
                                              /  |
                                            /    |
                                           O     V
                                     Main bearings

    Exhaust stroke (4)

    Exhaust gasses pushed out of cylinder by rising piston driven by inertia of flywheel (and blade).

        From ignition/magneto coil >========
                              Spark Plug  ||||
                _________________________| || |________________________
               |  _______________________| || |______________________  |
               | |                          =="                      | |
               | |     ->  ->  ->  ->  ->  ->  ->  ->  ->   ______   | |
               | |__ ______ ___    ->  ->  ->  ->  ->   ___ \____/ __| |
               |____\\____//   | _____________________ |   \  ||  /____|
                       ||  |   ||                     ||   |  ||         Exhaust
                       ||  |   ||=====================||   |  ||    ---> gases to
                ______ || /    ||=====================||    \ || ______  muffler
               |  ____||||___  ||=====================||  ___||||____  |
               | |     ||    | ||            ^        || |    ||     | |
               | |     ||    | || Piston   O |        || |    ||     | |
               | |     ||    | ||         /  |        || |           | |
               | |           | ||  _ _ _ / _ _ _ _ _  || |           | |
               | |   Intake  | ||/       /           \|| |  Exhaust  | |
               | |   Valve   | |        /              | |  Valve    | |
               | |   Closed  | |        /              | |  Open     | |
               | |           | |       /  Rod          | |           | |
               | |           | |       /               | |           | |
                  Crank pin journal  O --->
                                           O  Main bearings

  • Back to Small Engine Repair FAQ Table of Contents.

    Service Information


    The best reference is the manual that likely came with your engine. This will have the specific information needed to service it as well as the recommended preventive maintenance tips. Many engine manufacturers now have the user and service manuals for specific engine/equipment models on-line as well as extensive general maintenance and troubleshooting information, and parts identification and ordering.

    The following are good for general information but may not have the specific information for your equipment. However, the combination of the original user or service manual and a small engine repair book is probably all you need.

    1. Small Engine Repair, 2-12 HP
      Richard J, Rivele, Ed.
      Chilten Book Company, 1993
      Radnor, PA 19089
      ISBN 0-8019-8323-1

      (There are also several other versions of this book covering small engines up to 20 HP. For the most part they are very similar with some additional information provided where appropriate for each size range.)

    2. Chilton's Repair and Tune-up Guide for Small Engines, 1-1/2 to 20 HP
      Chilten Book Company, 1979
      Radnor, PA 19089
      ISBN 0-8019-6810-0 (Paperback: ISBN 0-8019-6811-9)

    3. The Complete Handbook of Lawn Mower Repair
      Paul Dempsey
      TAB Books
      Blue Ridge Summit, PA 17214

    4. Small Gas Engines: How to Repair and Maintain Them
      Paul Weissler
      Sterling Publishing Company
      Two Park Avenue, New York, NY 10016

    5. Simple Gasoline Engine Repair
      Ross R. Olney,
      Doubleday and Company, Inc., 1972
      Garden City, NY

    6. Walk-Behind Lawn Mower Service Manual - Third Edition
      Intertec Publishing Corporation
      P.O. Box 12901, Overland Park, Kansas 66212

    7. You Fix It: Lawn Mowers
      Carmine C. Castellani & Clifford P.Seitz
      ARCO Publishing, Inc., 1975, 1976
      219 Park Ave. South, New York, NY 10003
      ISBN 0-668-02705-3

    8. My Lawn Mower Hates Me - An Original Manual for Hand and Power Mowers
      Tom Cuthbertson and Rich Morrall
      10 Speed Press
      Box 4310, Berkeley, CA 94704

    9. Handbook of Lawn Mower Repair
      Franklynn Peterson
      Emerson Books, Inc., 1978
      Buchanan, NY 10511

    10. Repairing and Maintaining Yard Equipment and Power Tools
      Mort Schultz
      John Wiley & Sons, Inc., 1994
      605 Third Avenue, New York, NY 10158-0012
      ISBN 0-471-53500-1 (cloth:acid-free paper), ISBN 0-471-53501-X (paper)

    11. The Repair & Maintenance of Small Gasoline Engines
      George R. Drake
      Reston Publishing Company, A Prenice-Hall Company, 1976
      Reston, VA 22090
      ISBN 0-87909-724-8

    12. Small Gasoline Engines: How to repair and maintain them
      A Popular Science Book
      Paul Weissler
      Times Mirror Magazines, Inc., Book Division, 1975
      ISBN 0-06-014564-1

    13. Fix It Yourself - Power Tools and Equipment
      Time-Life Books, Inc., 1989
      Alexandria, VA
      ISBN 0-8094-6268-0, ISBN 0-8094-6269-9 (lib. bdg.)

    14. Small Engines - Mowers, Tillers, Chain Saws
      The Editors of Time-Life Books
      Time-Life Books, Inc., 1982
      Alexandria, VA
      ISBN 0-8094-3910-1 (retail ed.), ISBN 0-8094-3911-X (lib. bdg.),
      ISBN 0-8094-3912-8 (lib. bdg.)

    15. Small Gas Engines and Power Transmission Systems,
      A Repair and Maintenance Manual
      Walter E. Billiet
      Perntice-Hall, Inc., 1982
      Englewood Cliffs, NJ 07632
      ISBN 0-13-814327-7 (hardback), ISBN 0-13-814319-6 (pbk.)

    16. Garden and Yard Power Tools - Selection, Maintenance, and Repair
      Walter Hall
      Rodale Press, 1983
      Emmaus, PA
      ISBN 0-87857-446-8 (hardcover), ISBN 0-87857-447-6 (paperback)

    17. How to Repair Briggs and Stratton Engines, 3rd edition
      Paul Dempsey
      Tab/McGraw Hill, 1994, $14.95

      (From: Jim Nelson (

      This is an excellent book about B&S engines.

      I don't know any old galoots who can teach me this stuff, so I always end up at the bookstore. This book I had to buy to identify the missing controls on a B&S driving a mortar mixer I bought cheap.

    Links to engine and equipment manufacturers Try Doc System Small Engine and Power Equipment Resource list. (This is a copy found via a Web search. The original Weh site seems to have disappeared.) A local copy is archived at Sam's Copy of Doc System Small Engine and Power Equipment Resource.

    On-line small engine discussion group

    This is a free (well sort of, it is advertising supported which presumably means you probably have to tolerate banners and such but hopefully no SPAM).

    From the charter:

    "Small Engines is an open forum intended as a resource for individuals studying small engines, business owners, home owners or anyone for that matter. The basic purpose of the list is to serve as a forum for discussions and distribution of information. Subscribers are encouraged to post questions, comments, or announcements of interest to the list about small engines and which pertain to work or theory or ideas about the current development or past of small engines. Discussions include: electrical systems on equipment, what works and how to do things, the effects of small engines on the environment and a lot more."

    Check out: Yahoo Groups: Small Engines Discussion Group.

    Mower and engine parts sources

    Most of the common maintenance and repair parts that are likely to be needed are readily available and relatively inexpensive. Your place of purchase may be most convenient but not necessarily the cheapest.

    Many engine parts are fairly specialized and non-interchangeable unless the replacement part really is identical. However, even different model engines may use the same parts. For example, the identical connecting rod, piston, piston pin, camgear/camshaft and oil pump, valves, valve lifters, ignition components, and flywheels are used in several different 3-5 HP Tecumseh engines. Even the same crankshaft may be used in multiple models but slight variations like whether there is an auxiliary power take-off or not can complicate matters.

    Carburetors, magnetos and other ignition components from similar sized engines from the same manufacturer may be substituted in many cases. I have even heard of people adapting a larger carburetor to a small engine (with varying levels of success).

    Here are some of the alternatives for obtaining replacement parts:

    New parts and supplies - walk-in

    New parts and supplies - mail order/Web

    An increasing number of small engine manufacturers are providing on-line sales of repair manuals, general maintenance items, and "how to" books and videos. One example is Briggs and Stratton. However, they may not have more specialized items like head gaskets and connecting rods.

    There are a large number of parts supply companies. Only a few are listed below. (I have absolutely no affiliation with any of the following companies. Some of the descriptions are from the company's site or what they emailed me. Listed in alphabetical order, more-or-less.)

    Used parts

    It doesn't make a lot of sense to search the countryside for a clogged air filter or slightly worn engine oil :-) but it may be possible to save a substantial amount of money if you can locate a previously owned replacement for that bent crankshaft!

    Comments on buying used mowers

    Caution: Mowers older than about 20 years or so will not have the safety features of modern ones (dead-man bar for engine kill and/or blade-brake clutch). Therefore, it may make sense to avoid really old mowers without these safety features even if they are in perfect condition.

    Garage and yard sales, tag sales, flea markets, and auctions, are also excellent sources for tools. Very high quality tools often show up at reasonable prices but there is a lot of junk out there so know what you want and how to distinguish an easily salvageable tool from one that is a pile of congealed rust.

    (From: Mark Manville (

    What I did when we bought our first house was to just go around to garage sales and look for an inexpensive used mower. I figured I would need to cut the lawn for a while before I was really sure of what I needed anyway. Besides, after the down payment, closing costs, and other expenses, there was not much free cash to speak of. Such a mower could perhaps take you through the first year or so, until you have more experience, time, and cash. You may even get lucky like I think we did and get one that you can stick with for a while. We got a 22" Sears self propelled that works pretty well - it's old, but at $30 I count it as a bargain, even if breaks down after one year."

    (From: Erik Beljan (

    I would like to comment on buying used lawn mowers. Do not trust what you are buying. You never know what the quality of the engine is and there is no way to guarantee it. I found a Roper Rally 22 inch lawn mower last fall, and had an interesting ordeal which shows what you might be getting. I took it home only to find the engine was seized (found by attempting to pull the starter cord). I took the sparkplug out and put a mixture of Duralube All Purpose Spray, Marvel Mystery Oil, and Liquid Wrench into the cylinder. I put the spark plug in and let it sit a few minutes. I took the plug out and yanked the blade from the underneath. It snapped free. I then cranked it about 10 times, to clear the cylinder (if fouled the old plug that was in the mower) I cleaned the plug off with some carb and choke cleaner, sprayed some into the cylinder and carb. I proceeded to start it again. It kicked to life with a huge cloud of smoke. It blew smoke everywhere for about 5 minutes. I shut it off and parked it under a tree overnight so I could take a closer look the next day.

    The next day I took a better look at it only to find that the top of the flywheel was full of poplar tree cotton, which I removed. I looked at the oil which was a dark thick black color. I poured a large dose of Marvel Mystery Oil into the oil and started the mower. I ran it for a few minutes (it burned considerable oil), shut it off and changed the oil. I dumped the old oil out and filled it with a 50 50 blend of SAE 30 oil and Nu Lube oil stabilizer (a thickish oil treatment like STP thins out more though). I filled the gas tank and put about 4 oz of Marvel Mystery Oil in the gas. I started it and ran it for 10 minutes. No smoke, sounds like new. I am using it now this spring with the only modification of a new spark plug and air filter. The thing runs fine, but it is likely that the engine is in terrible shape internally (I am not bothering to take it apart, but can it can be seen by the excessive oil burning if I don't put some Nu Lube in the oil).

    If I sold it to you today without telling you what I did to it, you would never know, there are no outward signs of what it was like. The Nu Lube seals the clearances nicely, stops the oil from burning, keeps the spark plug from oil fouling, keeps the oil looking clean and quiets the engine quite a bit. If I were the purchasing party I would not want to receive this engine if I would have known its condition. This story goes to show that for minimal work you can make an engine run in so it seems to be in decent shape, even though it might not be.

  • Back to Small Engine Repair FAQ Table of Contents.

    Small Engine Specifications

    Typical mechanical specifications

    The following are some of the key dimensions and other specifications for several common Tecumseh engines. Where two numbers are listed, they are the upper and lower service (wear) limits as appropriate. One of these will be close to the expected dimension on a new engine; the other represents the point at which you are supposed to replace the part due to wear. All values are in inches unless otherwise noted.

            Specification        LAV/H30   LAV/H40   LAV/H50    V/H70    VM/HM100
    Horsepower:                     3         4         5        7         10
    Displacement (cubic inches):  7.75     10.5      11.5      15.0      20.2
    Bore:                         2.3125    2.625     2.75      2.75      3.187
    Stroke:                      1-27/32   1-15/16   1-15/16   2-17/32   2-17/32
    Timing Dimension:              .060      .035      .035      .050      .070
    (before TDC)
    Point setting:                 .020      .020      .020      .020      .020
    Spark plug gap:                .030      .030      .030      .030      .030
    Valve Clearance:               .010      .010      .010      .010      .010
    Valve seat angle (degrees):     46        46        46        46        46
    Valve spring free length:     1.135     1.135     1.135     1.462     1.462
    Valve seat width:              .035      .035      .035      .042      .042
                                   .045      .045      .045      .052      .052
    Crankshaft end play:           .005      .005      .005      .005      .005
                                   .027      .027      .027      .027      .027
    Crank pin journal diameter:    .8610     .9995     .9995    1.1865    1.1865
                                   .8615    1.0000    1.0000    1.1870    1.1870
    Connecting rod bearing diam:   .8620    1.0005    1.0005    1.1880    1.1880
    (crank pin journal end)        .8625    1.0010    1.0010    1.1885    1.1885
    Crankshaft diameter:           .8735     .9985     .9985     .9985    1.1870
    (cover/blade/PTO)              .8740     .9990     .9990     .9990    1.1875
    Main bearing diameter:         .8755    1.0005    1.0005    1.0005    1.1890
    (cover/blade/PTO)              .8760    1.0010    1.0010    1.0010    1.1895
    Crankshaft Diameter:           .8735     .9985     .9990     .9985     .9985
    (flywheel/magneto)             .8740     .9990     .9995     .9990     .9990
    Main bearing diameter:         .8755    1.0005    1.0005    1.0005    1.0005
    (flywheel/magneto)             .8760    1.0010    1.0010    1.0010    1.0010
    Camshaft bearing diameter:     .4975     .4975     .4975     .6230     .6230
                                   .4980     .4980     .4980     .6235     .6235
    Piston diameter:              2.3090    2.6260    2.7450    2.7450    3.1817
                                  2.3095    2.6265    2.7455    2.7455    3.1842
    Piston pin diameter:           .5629     .5629     .5629     .6248     .6248
                                   .5631     .5631     .5631     .6250     .6250
    Width of comp. ring grooves:   .0955     .0925     .0795     .0795     .0955
                                   .0977     .0935     .0815     .0805     .0975
    Width of oil ring grooves:     .125      .156      .1565     .188      .188
                                   .127      .158      .1585     .190      .190
    Ring end gap:                  .007      .007      .007      .007      .007
                                   .020      .020      .020      .020      .020
    Top piston land clearance:     .0105     .0165     .024      .023      .029
                                   .0145     .0215     .027      .028      .034
    Piston skirt clearance:        .0025     .0045     .0045     .0045     .028
                                   .0040     .0060     .0060     .0060     .053

    Typical torque specifications

    For proper operation and long life, you cannot just tighten critical nuts and bolts by feel. You really don't want the connecting rod cap to come loose while the engine is running! A Web site that has torque specifications for many popular engine models as well as other useful information is Outdoor Power Equipment.

    The following are just samples - check your engine manual for exact values!!! The most critical are the connecting rod bolts or nuts and the cylinder head bolts.

         Location                    Torque ---> Inch-pounds    Foot pounds  
    Cylinder head bolts:                          160 - 200       13 - 16
    Connecting rod bolts (Durlock rod bolts):
          2.5 - 4 HP small frame:                  96 - 110      7.9 - 9.1
            5 - 6 HP medium frame:                130 - 150     10.8 - 12.5
            7 - 10 HP medium frame:               150 - 170     12.5 - 14.1
    Cylinder cover or flange to cylinder:          65 - 110      5.5 - 9.0
    Flywheel nut:                                 360 - 396       30 - 33
    Spark plug:                                   180 - 360       15 - 30
    Magneto stator to cylinder:                    40 - 90       3.3 - 7.5
    Starter to blower housing or cylinder:         40 - 60       3.3 - 5.0
    Housing baffle to cylinder:                    48 - 72         4 - 6
    Breather cover to cylinder:                    20 - 26       1.7 - 2.1
    Intake pipe to cylinder:                       72 - 96         6 - 8
    Carburetor to intake pipe:                     48 - 72         4 - 6
    Air cleaner to carburetor (plastic):            8 - 12           1
    Tank plate to bracket (plastic):              100 - 144        9 - 12
    Tank to housing:                               45 - 65       3.7 - 5
    Muffler bolts to cylinder:
           1 - 5 HP small frame:                   30 - 45       2.5 - 3.5
           4 - 5 HP medium frame:                  90 - 150        8 - 12
    Electric starter to cylinder:                  50 - 60         4 - 5

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