How to Repair a Lawn Mower Engine.

Picture of How to Repair a Lawn Mower Engine.
In this instructable I will show you how to repair a lawn mower engine, and how to finally kill those gremlins which have been lurching in its bowels. Of course there is now way I can tell you every single problem that could happen to your engine, so I will tell you the major parts of the engine, and how to repair them. I am not a trained mechanic. What I have learned comes from years of reading, and much trial and error. Hopefully from that I can teach you how to become more of an independent person, and maybe save you a few bucks
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Step 1: Disconnect the spark plug.

Picture of Disconnect the spark plug.
4 stroke 10.JPG
Before you do anything with the lawnmower you should disconnect the sparkplug. This will leave no chance of the engine starting with you working on it. It is better to loose a few second than loose a few fingers! It is also be a good idea to wear some working gloves, because engines can be very sharp and dirty which will give you an infection. Eye protection should also be worn so that gasoline and bits of metal cannot end up in you eye.

Step 2: Pre Diagnoses

Picture of Pre Diagnoses
Before you go and disassemble the whole engine it would be a good idea to first look at the spark plug. This can give you important clues about what is going on in there. The one spark plug condition that was not mentioned on the chart is the meltdown. The sparkplug will have melted metal around the gap, and is not typical but can be caused by a lack of good oil in your engine(CHANGE YOUR OIL). As long as I am talking about spark plugs I should also mention that if your spark plug gets striped you will need to buy something called a heli coli, which will patch the damage.
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Jjavaheri1 month ago

This is a great post! thanks for posting it

hokiegirl4 years ago
There is oil leaking around the engine case of my push mower and it won't start. When it does start, runs very, very rough and only for less than a minute with lots of smoke and then shuts down and cannot be started. Any ideas what might be wrong? Pam
tilt over the mower and look at the center of the blade. pull the starter cord slowly and see if the head of the bolt wobbles as it turns. If it does you have bent the crank and the engine is good for parts only.

If that's not the case, check to see if the flywheel key is partly broken. This could mess up the timing enough to cause your issues. It's a > $5 part.

Stop! Safety first! Before attemptig the above instructions it is important to first remove your spark plug. Also when you do tip your mower it is important to only tip it with the carburetor side up in the air!!!!

If the choke is off and still blowing lots of smoke. is it a 2 stroke or 4 stroke? have you cleaned the air filter any time that you have owned it or the oil for that matter. It sounds like a cylinder or head gasket to me. Its sucking in air . after piston ignition, hence all the unburnt fuel. OR It could be something so simple as a stuck float in the bottom of the carby thats giving the thing too much gas. like having lots of booze put down ya guts, its gunna hurt
too much oil, try draining the oil and replacing it with the right amount, keep checking the dipstick as your adding oil
thundercookie4 months ago
just what I was looking for
Tbus6 months ago

This is a good learning tool to repair a lawn mower. I just purchased a repair book that has a bunch of "how to" pictures and is actually a little self-paced course with a CD. It's a nice reference tool if you ever get in a repair bind:

This is a very good instructable, however, as a small engine mechanic I don't recommend pulling a flywheel with a crowbar. Flywheels are usually cast aluminum and can crack or warp if you use a crowbar, a gear puller should always be used to pull a flywheel. Heat applied to the aluminum surrounding the crankshaft can help free a stubborn flywheel.
mercier blue11 months ago
briggs mower wont start ,blowing oil out of muffler.any thoughts
If it is blowing oil out of the muffler it probably has a bad or broken valve which would cause it to have low compression and not start if you have a compression tester that would be a good place to start
I waited until I had a full season of use before submitting this review. My prior mower was an electric corded BD Lawn Hog that I had for six years. I'm used to being tethered to an electric cord for mowing the lawn. My property is 60' x 100' on Long Island and relatively flat. This Greenworks mower is way better than the BD I used to have. It's a bit bigger, 20" vs 19", but it's lighter, more manuverable and even though both are 12 amp, the Greenworks mower seeems to be more powerful. I always bag the clippings so I can't comment on how well it mulches. The bag mechanism is great. The spring loaded flap closes securely. The deck is metal not plastic as in the BD. The blade design is different too and I think it cuts better. I did the lawn today after not cutting for two weeks. The Greenworks cut through the high grass with ease. The price is great. It's a "green" product that doesn't pollute the air. It's quiet compared to a gas mower. The light weight makes it very easy to use. I have no complaints am I am very pleased with this product. Read more here
knowlzys9 months ago
we have spark, it was overfilled with oil and fixed this; -Briggs and Stratton- put in new petrol and now no start?
diva1911 year ago
i removed mine, can not get it back on. my problem was grandson put oil in gas tank. any suggestion.
rinse with water and soap to get the oil out (be sure to get all the soap out) then rinse with gasoline
mattyfu1 year ago
Great ible, thanks!
PaulRedd2 years ago
I signed in for the free account and I have tried three times to get this pdf and I keep getting a loop back to where I was.
adel antado2 years ago
My father told me that an ounce of prevention is worth a pound of cure. The majority of lawn mower problems relate to old gas, that is gas left in the mover at the end of last season then expected to start at rhe beginning of the next season. Gas is hydroscopic, that is it will absorbe moisture from the air. Over the winter season, any gas left in the mower will absorb enough moisture from the air to make it uncombustable. The "ounce of prevention" for lawn mowers is at the last mowing of the season is run the mower until all the gas is used. Then, open the cap of the small tank next to the carborator to let any residual gas evaporate. The mower will fire-up as new with fresh gas at the beginning of the next season. This should be done after every mowing in Florida where the humidity ruins gas in a few days.
Also, it is a good idea to change the oil before beginning to use the mower in the new season. The air filter also needsw cleaning or replacing each year. A new spark plug every year ore two is also a big help.
wrmc2 years ago
I have a Toro 1987 mower with a great aluminum base. It is still in great shape. And it is light!! My engine is shot. I would like to replace it with a new engine. The motor was a Tecumseh 4.0 hp. wit a 1" by 2" shaft. I don't find any engines with this same size shaft for sale. the closest I find is a 1 13/16 by 7/8 ". Tecumseh is no longer in business. Toro doesn't sell a new engine for it. I have found engines in this shaft size that are meant for power washers! Will they workon a lawnmower?

It appears that all small lawn mower engines have a standard attachment hole configuration in the base of the mower. That is except for Toro. It looks like the standard is rotated by 45 degrees from the engines now for sale. Is this true?

Will a new engine mounted side ways on this configuration work ok?

Can the manual start mechanism be installed so that it is in a different position so that it will come out of the new engine in the proper position to be pulled from the back of the engine?

Sounds like a lot of problems to be worked around!!!
bdaoud3 years ago
I have a problem with my lawn mower and that is when I turn it on, it releases black smoke and sputters then eventually it turns off. It has plenty of gas and I thought the problem could be because it was low on oil. I put enough oil in it but it does the same thing. Please help.
sady1991 bdaoud2 years ago
You may have too much oil in the mower
Sounds like the air/fuel mix is too rich. Check that the choke isn't on and that there are no blockages or broken parts in the carb and that the butterfly valve is turning with the throttle.
ashtiany2 years ago
I have craftsman with kawasaki FJV series engine. Problem i have after changing the spark plug, primb bulb and clean the carburator engine start right away and run run for 2 seconed and shut off. by pumping fuel via primery bulb start again and run for 2 seconed and shut off again. how can i fix my problem.
Craftsman 197CC with Kawasaki FJ180V-FS05 series motor
thak you
rsowalker2 years ago
I went to start my mower for the first time and it pulled funny and oil came out of the muffler. What do you think
sw2542 years ago
im haveing trouble with my lawnmower when i pull it and it clutters i checked the spark plug and it sparks so im not sure whats wrong any advice
notjoe92 years ago
Hello All,New guy here,having trouble with 12.5 briggs I/C motor (model 289707 / type 015401 )
it will not start,has good spark,i shot ether in back of carb. flame came out,so I am wondering does this prove that I have a timing/valve problem? or could this happen from a dirty carb?
Thank you,
rm88 notjoe92 years ago
notjoe9 - Smoke or fire tends to be indicative of burning oil. As far as general Briggs engine troubleshooting, they have a page about the reasons why engines do that and a few possible causes. Check out their FAQ:
Racer2312 years ago
Im trying to learn how to build one so i can fix one please teach me how to build the engine. i dont know wats up with my engine so i wanna take the newer parts and build a new one.
ichevez2 years ago
Richard,  if you are still having trouble fixing the small engine, there is a great shop in Baltimore where they fix all kind of small  diesel and gas engines. This is the link to their website: Small engine repair.

Great article btw, let me know if you have any other questions.
34638Fred4 years ago
 I have a 6.5 h.p. briggs that i can pump the bulb 3 or 4 times and the engine will start but shut off immediately. It sounds like it runs out of gas. but when i take the carb apart the bowl is full and the float moves freely. It seems to be a carb problem to me. I have taken the carb off and filled it with carb cleaner and blown it out with air. It has a brand new air filter and new gas. Can anyone shed some light to what might be the problem. Any comments will be appreciated 
i think your injecters are stop up try clening them out
There are no injectors in a lawn mower engine

If you haven't tried this, take out the main jet ( doubles as a screw) that unscrews when you remove the bowl.  Then get a little wire and push it through the top and the bottom of the jet as well as the little holes on the sides.  When the jets get clogged, the mower can't breathe.
When the jets get clogged....fuel cannot be sucked in and sprayed into the fast moving air stream (low pressure Bernoulli effect at the jet opening into the venturi/throat) of the carburetor; ... Over winter, water condenses from humidity in the fuel tank with cycles of temperature changes...denser than fuel it sinks to the bottom and is then first to head to the carburetor's float bowl; corrosion ensues in the tank....and in the float bowl if not old fashioned glass (which was glass for a reason--see water and debris for easy removal); additionally with aluminum allow float bowls, the corrosion is somewhat chalky and can be sucked in and wedge in the jet (as well as the steel gas tanks iron oxide/rust grit); not to mention the dissimilar metal potential for electro-chemical corrosion.... careful with the wire, jet precision size determines flow, hence different number/size jets for elevation and engine/carburetor flow capacity; copper wire from multi-strand electrical wire/lamp cord will often be small enough, and fairly soft...

if your lucky it is just water to empty from the float bowl: water and/or corrosion debris, is the common reason for brief run and shut down, after squirting a bit of fuel through the spark plug hole before replacing spark plug (after have checked for such a bit of debris or carbon in the spark gap, and if spark to check magneto)

And for electrical, just lay the spark plug (connected with the spark plug wire) on the engine fins (for ground connection), pull the starter rope and look for the spark..
A quick test would be to

!. take the air cleaner off (but return the bolt to the hole, and screw the bolt in part way.

2. start the engine after priming per normal

3. give it a shot of starting fluid or WD-40 down the throat of the carb, if you can keep it going with a spray bottle, then you have a fuel problem and can rule out ignition or timing issues

I'm guessing you either need to re-clean the carb or there is debris in the fuel tank that block off the fuel after it it gets started.

Does the bulb return to "inflated" as soon as you release it, or is it a little slow?
Someone PM me about this comment and I thought I'd share my reply.

>>You asked if the re-inflate bulb on a briggs & straton engine was slow to reinflate. That is happening to me and I'm guessing it is from restricted air flow into the carb.

remove the air cleaner, spray with carb cleaner, reassemble at least the center bolt that holds the air-cleaner on (important for many engines). Do a $1.98 tune-up - This is detailed by me in the comments here: I

n your case, you'll need to prime, start the engine, and slap your hand over the air intake before the engine dies due to lack of fuel. You might be able to quickly remove your hand and spray into the air intake some carb cleaner or wd-40. to keep it going. It sounds to me like you have a fuel delivery problem. You can verify it's the fuel by starting it and keeping it going with sprays from wd-40 down the carb throat.

I'd also check and see if there's anything in the fuel tank, and wash it out if need be. If there's a fuel filter, it may be worth trying to replace it.

Dave Rock has quite a few videos on small engines. Try going to

and then use the search box with the term "briggs" to tease out the good stuff from all the track racing and car jumping vids. Good luck.
I have the exact same engine and same problem. I checked my carb. and it was dirty, cleaned it and still nothing. The magneto is probably the culprit, but I am not positive how to check it. I t shows a short with and ohm meter when I touch the ground wire and the engine. Any idea? thanks

KJ6EPL3 years ago
I would also like to add that another problem that makes the engine hard to start or not want to run at all is a worn out carburetor bowl gasket. It will drain your gas tank quickly too as gas will evaporate out of the carb. Sometimes gunk can help seal up an aging gasket to the point where the gasket will not seal once cleaned. To test for this, with the engine COLD and OFF (if it's hot/warmed up a fire may result), connect the carb to the gas tank and allow the bowl to fill. Tilt the engine slightly. If gas spills out of the carb bowl, replace the gasket.
Beach633 years ago
I have a Murry lawnmower with a Techunsia 4.4hp engine. It starts for about 2 seconds then cuts off. Anyine have any ideas?
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