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.
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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.

Step 3: Ignition System

Picture of Ignition System
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The most common ignition system is the magneto ignition system which is kind of like a step up transformer. The most important thing about a magneto system is that there needs to be a small gap between the magnets in the flywheel, and the magneto. The flywheel itself is turned by a pull of the starter. There should be two teeth on the starter so it can turn the flywheel, and when the brake is disengaged the cord will rewind. The gap between the flywheel and the magneto should be no wider than the thickness of a playing card. There are many other ignition systems which would take longer than this instrustable to explain, but as a rule you should check all the wiring and look for any wear or grime that might cause problems. You can easily test the ignition system by touching the end of the spark plug to a grounded piece of metal, and see if there is a spark when you start the engine. Do not hold any metal on the spark plug or you will get a painful zap.

Step 4: Removing a Flywheel

Picture of Removing a  Flywheel
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Removing a flywheel can be a aggravating process if you do not know the tricks. The nut which holds the flywheel is on very tightly, and can only be removed by blocking the rotation of the crank shaft with a wrench holding the nut on the blade or by blocking the rotation of the blade. Do not try to hold the flywheel by sticking a crowbar in the teeth, or they might break off. Once you have remove the nut which holds the flywheel in place you will need to gently pry off the flywheel with a crowbar. Keep rotating the flywheel until it can be lifted off easily. Now put the flywheel key somewhere safe. When you want to put the flywheel back on make sure that the key is in the right direction. It is usually marked in some way.

Step 5: Cleaning the Carburetor

Picture of Cleaning the Carburetor
The Carburetor is the number one culprit with engine trouble. It such a important part of a 4 stroke engine that I will have to use two pages to fully explain how to clean it. The number one thing about repairing a carburetor is that they must be very clean. Here are two examples of very common carburetors which you can find on a lawnmower.

Step 6: Cleaning the Carburetor

Picture of Cleaning the Carburetor
As I said before the carburetor must be very clean in order to work. If a carburetor has had gasoline stand in it for more than a year it will gather a gunkey varnish which will have to be remove. This gunk can be removed easily removed by letting the metal parts sit in a jar of gasoline over night. Do not let the plastic parts stay in gasoline for to long or it can rune the plastic. Now for the individual parts starting with the float. To examine the float you will first have to remove the pin making sure you do not bend it. To check to make sure the float is working properly submerge it in water to make sure there are no holes. The float valve itself should be able to move freely and once again needs to be clean. The most common problem with a carburetor is that its jets have become clogged. They are usually made of brass and are sometimes removable. To clean them out simply run a small piece of wire trough them. The main carb body itself has many holes which can also get clogged from time to time. Use a flashlight or a laser to make sure every hole is open, and clean them out with a piece of wire.

Step 7: Fuel tank and Lines.

Picture of Fuel tank and Lines.
If the engine is having trouble starting there is a chance that the hole in the gas cap has become clogged (If your engine has this feature).To check this just lightly screw on the cap, and see if the engine starts. If you need to just run a small gauge wire through the hole to clean it out. When there is only a trickle of gas coming out of the main line you should check the gas tanks filter, which can sometimes be cleaned a little with a light brush. If the gas is still coming out slowly try cleaning the lines this can be done by running a pipe cleaner or something similar through the line until the pipe cleaner comes out clean. Some engines also have push primer which helps create the right air fuel mix for the engine. It should be free of cuts and the air hole should be open.

Step 8: Intake and Exaust valves

Picture of Intake and Exaust valves
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There are two valves in a 4 stroke engine. The intake valve which injects the air fuel mix into the combustion chamber, and the exhaust valve which lets out the carbon monoxide and what is left after the combustion. These valves are moved by a cam shaft that turns with the engine at a set time. Sometimes these valves will gather junk around the shaft, and will need to be cleaned by a commercial valve cleaner or by hand. The valve should look very nice and feel smooth in order to work. Sometimes the valves will get scratch and leak oil. This is mostly due to not changing your oil. There is nothing that I know you can do once the valves get this bad, and the only thing you can do is buy new ones. The valve heads can also get chipped and will inject fuel at the wrong time and reduce combustion. Once again if the valves get badly chipped you will have to buy a new valve. To remove the valves you will need to press down the compression springs, and remove the valve spring retainer. Now, putting the valve parts back together again can get a little tricky .You will have to first put the spring and the valve spring retainers back into the engine together. Once you have done this you will have to insert the valve back in place. To place the valve spring retainers back on the Valve you will need to press the valve spring retainers a little bit to the side so you can push the valve into the large hole of the valve spring retainer. Then you will need to press the spring and retainer down so you can lock the valve spring retainer in place. Some valves also have something call a sleeve which you will need to very delicately put on top of the valve and below the valve.

Step 9: Spark Arrestor

Picture of Spark Arrestor
The spark arrestor is not a very important part of a engine, but a part that is required by law on all outdoor engines. As its name suggests it catches sparks that come out of the engine and prevent fires. Most spark arrestors cannot be opened and can only be cleaned with a wire brush which will break up most blockages.

Step 10: Removing the Piston

Picture of Removing the Piston
To examine the piston you will have to separate the two halves of the engine. Doing this can be dangerous to your engine which I would not recommend. When you separate the two halves do it very slowly prying all around the engine. Once you have separated the two halves disconnect the piston from the crank shaft. Now push the piston out through the combustion chamber. First check to see if the gaps in the rings are lining up when you pull the piston out of the engine. If they are there is probably nothing wrong with your piston, and all you will have to do is align the piston ring gaps at about a 90 degree difference so that the oil cannot leak through. If this is not the case check the cylinder walls for scratches which could be letting in the oil. After you have done both these steps and still find no error you can the assume that your piston rings have gone bad. To remove the rings a tool called a ring compressor should be used, but two pairs of pliers can also work(just do not push the rings to far).It is very important that you do not put the wrong ring on one of the groves. Once you have placed on the new rings you should scatter the gaps in the rings ( I like somewhere around a 90 degree difference) then give the rings a little bit of engine oil so they go in easily. Before you put the piston in make sure it is in the right direction according to the markings. you may need to squeeze the rings a little to get the piston into the cylinder. Now you simply need screw everything back together

Step 11: The Conclusion

Picture of The Conclusion
See, I can be sophisticated. If you have done any of these steps there is a big chance that you might have damaged one of the engines gaskets and will need to buy a new one. I hope you have enjoyed my instruct able and that maybe it has helped you in someway.
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wonderful.. I really like it...

Kilo mike4 months ago
I was mowing my lawn and hit a ant pile and my mower stalled out and wouldn't restart any suggestions on what it could be
RoseH15 months ago

Thank you for such a detailed tutorial. I appreciate the included pictures along with the explanation. Sometimes I hear the words people are saying, but I can't picture what they are talking about. Our lawn mower has been acting up lately so I'm going to run through these steps to try and get it working again.
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I have no strength on my arms and can't pull the string hard enough to ignite the spark plug of my mower. Can the spark plug be replace with a battery clicker...? It looks to me that the only thing needed is a spark to start the engine running.... do such gadget exists....? Pardon my ignorance.... :)

I have no strength on my arms and can't pull the string hard enough to ignite the spark plug of my mower. Can the spark plug be replace with a battery clicker...? It looks to me that the only thing needed is a spark to start the engine running.... do such gadget exists....? Pardon my ignorance.... :)

The choke on my Craftsman self-propelled mower gradually shuts itself off after starting, but when it does, the engine stops. If I hold the choke lever about midway and not let it shut off all the way, the mower continues to run. Any suggestions? It's 10 years old, but I haven't run it for 4 years. It worked fine then.

Hello Bill,

Sorry a late response..But if its been sitting that long its having a hard time drawing the fuel up from the bowl of the carburetor..I suggest taking the carb. apart soaking it in a chemical carb cleaner for a couple days..Its most likely gumed up inside there like tar it will never do right until that is done.Then blow it out with air hose and a spray cleaner you should be good to go.

Thanks for the help. Running it with the choke half on for now.
Lawn Mower is a machine which is used for cutting grass in garden, lawn etc. The machine is very helpful in cutting in equal size & shape. This equipment is generally used in houses, bungalows, resorts etc to create beautiful scenery for visitors. The tool requires daily maintenance due to work properly. Here some steps are written which can help user to repair the machine. Similarly the cars should be serviced in proper service center.

The battery died on my Lawn Mower, so I put Gas in it ,I primed it to start, when the gas is used up fron primer the mower dies. what can I do/

BruceA17 months ago

Have a question on a mower..Go to and click on contact button ,ask your question and will reply as soon as possible!


Jjavaheri9 months ago

This is a great post! thanks for posting it

hokiegirl5 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
just what I was looking for
Tbus1 year 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:

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
knowlzys1 year 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
mattyfu2 years 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
sw2543 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
notjoe93 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 notjoe93 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:
Racer2313 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.
ichevez3 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.
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