Introduction: 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

Step 1: Disconnect the Spark Plug.

Picture of Disconnect the Spark Plug.

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

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

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

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.


fred2strk (author)2017-10-12

i pulled a really great boner move working on a briggs motor on a mower. I thought

the engines was seized since i couldn't pull the coil start so I took some youtube

advice and put some marvel mystery oil in the cylinder. After doing this I realized

that i had simply not released the engine brake and the motor turns just fine but oil came out from the exhaust. How do i get the MMO out of the engine. Also the engine makes like a honking sound when I pull the cord does that mean anything ?Any sage advice would be appreciated.

waaaaste (author)2017-06-24

I have a Kohler lawn mower that ran well for the first five minutes and then cut out. I've had that problem before with a few other machines and it was a sign the coil was in trouble. I bought a new coil and again it ran flawlessly for the first five minutes and then died. Does Kohler have a problem with their coils?

TahseenE (author)2017-04-20

I have a Husqverna lawn mower (LC 121P) with Briggs & Stratton engine. After changing the engine oil which I bought at Home Depot, the engine started with white smoke and then stopped. It did the same thing after a few attempts to start it and then it completely stopped. Since then I can't even pull the starter rope to try to start it. I checked the filter which is soaked in oil. Any advice will be highly appreciated. Thanks!

David Solberg (author)2017-02-28

Really good instruction! Actually we can repair any batteries Thank you Tyler166!

mjuarezcna (author)2015-06-13

My lawnmower is a Briggs &Stratton 300 series with a 4.5 . I recently have been having trouble with it. The primer seems to have no pressure and when I check the filter its soaked with gas? What do I do?

TylerB166 (author)mjuarezcna2017-01-28

Your primer line may have a crack in it or have air in it

lcjrs (author)mjuarezcna2015-10-06

Sounds like to me bad fuel,carburetor could be dirty where the air is suppose to go to the primer,sparkplug could be bad, and/or rust on the flywheel from my knowledge.

BestLawnMowerR (author)lcjrs2016-05-21

I agree

BestLawnMowerR (author)lcjrs2016-05-21

I agree

mick916 (author)2016-07-21

I haves Briggs motor took apart carb and cleaned it new plug it will only start for a second or two if I squirt some starting fluid in carb then I dies out

TylerB166 (author)mick9162017-01-28

Your jets are not working most likely

LauraC183 (author)2016-08-02

white smoke was coming out of the exhaust. I checked the oil and it was low so I put a little bit back in and checked it again. Started it, still smoke a little bit. as I started to mow... all of a sudden it started billowing white smoke. I let go of it so it would turn off and for a few seconds it continued to sputter before finally turning off. Now I see oil on the body near the filter. Not sure what the problem is. Its only about a year old.

TylerB166 (author)LauraC1832017-01-28

You may have water in the engine. If the smoke turns black thats fine but white smoke mostly means theirs water in the engine

DorothyT8 (author)2016-08-14

My self- propel function us not working. Cable moves normal heading into housing, front wheels move fine. Mower starts right up, but I" m pushing manually..

Galgenstrick (author)DorothyT82016-10-08

Gears on the wheels are on backwards. Or drive belt is too loose

CliffS20 (author)DorothyT82016-09-11

Cable might be broken in the cable housing

England1956 (author)2016-08-30

I had oil on piston finialky cranked then changed ran shaking then started smoking from top of motor under rope swealing then slowly cut off what happen and what do i do to fix it

LawnMowerGuide (author)2016-08-25


AndreC123 (author)2016-07-31

I have a Craftsman Brigs & Stratton with 4.75 Gross, recently when we start it, the engine begins but them almost immediately turns back off. We gave it a tune up, but there's still no difference.

JamesM480 (author)2016-05-14

Whatever you do, don't buy any mower made by MTD. As for the battery operated rechargeable mowers? What an absolute joke . . . sure love the environment as you are forced to buy new batteries every year (do you have a clue as to the environmental impact of making these batteries . . . your better of burning diesel and it will also cost far less). We've all got a pile of "expired" rechargeable power tool batteries in our basements . . . This is like the old razor industry or printer industry rule . . . buy our junk for cheap and we'll make a killing on selling you short-lived razors, ink cartridges and power tool/mower batteries.

Bumbling Bee (author)JamesM4802016-05-17

Sure my MTD plastic carb warped. But I just put foam seal around it to seal where it mounts the tank. Cost me one PENNY. I even have a spots of rusty gas tank. So in 9 years I still haven't had ONE up/down rev'ing motor problem so common with these cheap carbs. No problems. No more carb kits because it's a carb warp gap problem and not a dirt problem. Just keep a carb kit handy if the spray foam seal doesn't solve your diaphragm pump issue that causes the up/down engine rev'ing. The MTD 3.75HP? mower is 15 or more years old now and works just like new with 15 year old original diaphragm gaskets. It's a seal problem not a dirt problem 99% of time.

Jamiejett (author)2016-04-30

thank you, thank you, thank you. you helped me fix my lawn mower.

gramps46 (author)2016-01-21

OK its not a lawn mower but its fuction is similar. honda gx160 driving a waterpump has a fixed jet carb. origina carb missing so i bought a jap cone. the problem is it starts/runs fine on choke but if i take choke off even sightly it dies out! took carb apart to look for problem everything seems fine. i happen to come across a youtube vid by a kid who had similar motor and carb and he had the SAME PROBLEM!? i also was able to get full throttle to work by manipulating choke. so it seems to be completely missing low/midrange. i drained float bowl and it had plenty of gas in it.

DezS2 (author)2015-12-08

Can anyone help me with what they use to seal between the air box and carb intake on a honda gxv120 engine. Seems they used a black gasket sealant would this be right? Tia

BruceA1 (author)2015-11-12

Keep in mind some of these small Honda engines,Carburetors,Like the GCV 160 If leaking out of the air filter box.. alot of times for me with experience of many years working on these ends up being NOT the needle valve itself but up inside where the rubber tip meets the aluminum body of the carburetor, If it has some age on it or been outside moisture causes corrosion and although you may try a new needle valve which they are sometimes up to 10.00 to purchase its 80% likely the upper seal, aluminum port hole.and you will need to replace the whole carburetor,I have a trick that sometimes works to keep from having to buy a new carb, Which involves copper piece of wire not to stiff also not to weak on a slow speed dremel tool ..At your own risk to try but I look with a jewelers loop and bright light to see this corrosion before I even attempt, I also use this loop to look at the rubber tip for rot or a deep ring on the rubber tip.So just some info while I have a moment to help anyone out thats having this problem..Now on a Briggs & Stratton carb..5 to 6 H.P. These are different..You can just buy a kit that has the rubber seat up inside of its body which also comes with the needle valve.Item # 398188. If you have any questions Send me an E-mail and will try and help further.



TheDIYman (author)2015-11-02

It`s nice to know that one can find a site that won`t make one jump through loops and bounds to get information only to find out that you have to pay for the information. Most of all, information that is jumbled. This is the most presented information that is simple and direct to the point. Thanks.

jap01886 (author)2015-04-02

Thank you for tutorial which is very helpful to me as I am a woman. I wonder if I could ask you a question. I have a Honda GCV160e lawnmower engine. I put new oil checking that it was up to the top of the gauge at the start of the season and checked the air filter and spark plug. It started and I was very pleased. I noticed however that it was using an awful lot of petrol. I then noticed that fuel was coming through the air filter and exhaust. The carb was cleaned thoroughly last year by a professional so I was surprised that it could be a stuck float in the carb. I took the carb off and reseated the float and put it back on. I now find that I cannot deploy the cord. I have had the cord mounting off and it works fine when not engaged on the engine. The break works as I have deployed the dead man's handle and the lever goes backwards and forwards. I have taken the spark plug out to see if I can move the blade but I cannot. Can you please help as I am a widow and my husband would have probably known what to do. I didn't hit anything prior to me noticing the petrol coming through the filter. I made sure to tilt the machine the correct way as this was instilled into me by my husband when I cleaned the underside. Any help would be appreciated. Thank you.

I'm at the same point you are. There is probably gas in the spark plug. That's what I'm trying to fix now, got the gas out and now it pulls but I don't think I'm getting spark. I had worked on my carb too and that's how I need up getting gas in my spark plug

Hi Mark, thanks for your comments. I have actually had it going again but the same thing happens I get fuel coming out of the air filter and exhaust. The float is working because I have attached a tube to the carburetor where the fuel goes and poured water into it and when you deploy the float up or down the water in the tube either goes down or stays in the float. I think my problem is associated to the linkages. I cannot when I deploy the choke see the linkage move by the carburetor. It moves further down but not by the carb, so I am thinking it might be a spring issue with me but I cannot see where the spring goes on the control lever. Basically it starts it runs for about ten minutes and then I get a load of fuel coming through the air filter. When I changed the oil last time it had a lot of petrol in it. I hope you sort your problem out but to be honest I have changed the oil in mine now three times and it is an expensive business. If you cannot pull your cord just keep pulling with spark plug out. It is called hydro something or other but will eventually get easier. I manged to get mine going by turning the blade which was locked solid to begin with but a little knock with a hammer loosened the whole movement, and when I turned the blade you could hear oil splurging out of the oil tube. Good luck!

lcjrs (author)jap018862015-10-06

That something is hydro lock

rmelchiori (author)jap018862015-04-24

The float pivots and moves a tiny needle valve that rests in a tiny seat. Sometimes a little piece of dirt will get lodged there and keep the needle valve from seating and let the fuel continue to flow. Since you had the carb apart it is possible that it is missing or damaged. A drop to the floor can easily damage the trip of the needle valve and keep it from seating. Lawn mower carbs are fed by gravity. Since the tank is higher then the carb, if the needle valve doesn't close, it will continue to feed fuel and eventually overflow into the cylinder. Once enough gas gets into the cylinder, you are not able to pull the starter rope because of the gas in cylinder, which can not be compressed, stops the pistons travel. Also, floats are set to a very specific level, which vary by carburetor. If it is not set properly, it won't have enough pivot travel to close the needle valve.

jap01886 (author)rmelchiori2015-04-24

Thanks for your comments. I have checked the float and the pin and it is engaging correctly. The experiment with the water down the fuel tube on carb and then moving the float up and down shows that it is working correctly. Someone had a quick look at it yesterday and they said that when the choke is engaged the control lever moves but when you move the choke back to its resting position after starting it, the control lever on the carb is only moving about 65% which may be my problem. I have been told to manually move it back into position and see if this works. It might also explain why it takes approximately ten minutes before the fuel leaks through the air filter. Thanks for your help!

lcjrs (author)mark.williams.7132015-10-06

Check flywheel and magneto/ sparkplug wire on the inside of the mower for any rust.

lcjrs (author)jap018862015-10-06

If the mower is pretty old it sounds like internal engine damage from normal wear and tear of the mower. If it's a newer one that's still under warranty I would have them look at it just to be safe.

mamak_2 (author)jap018862015-05-24

I think the instructions are helpful for both men and women (wink). However, I think that, unlike many instructions, these are written in a way that both men and woman can understand and that takes talent! I am sorry for the loss of your husband but so happy that you are figuring it out for yourself!

isaiah.wicoff.3 (author)jap018862015-05-06

You could have a bent or worn crankshaft.

jap01886 (author)isaiah.wicoff.32015-05-07

Thanks for your comments but it had hydro locked. It is now working but I still have the same problem with fuel coming through the air filter after about ten minutes. I have been told that the choke after being engaged isn't going back to its resting position properly and to manually move it the last fifteen percent.

bharris-gregory (author)jap018862015-04-27

Mine wouldn't start. I was told to smell the air filter. Smelled like gasoline. My advisor asked if it was a white filter. I said, "no." He told me, "It should be! Change your filter!" I did, and it started right up. You can also check that, by trying to start it, with the filter out. If you take it out and it's probably a clogged air filter. Easy fix. I hope that helps.

jap01886 (author)bharris-gregory2015-04-27

Thanks for your comments. It has had a new air filter and it still does it. I am convinced it is not the filter or carb but something to do with the linkage no engaging fully in resting position. I will try to manually move it the last bit after starting and see if this helps, otherwise I am at a loss. Thank you>

aseymour560 (author)2015-10-05

Sedge great instructable just cleaned my briggs and stratton mower fuel tank and carb starts fine but " hunts" and as soon as i try mow it cuts out . When its running there is a spring loaded valve that keeps moving which drops the revs then goes up does this a few times before cutting out....any suggestions please....Andy

aseymour560 (author)2015-10-05

Sedge great instructable just cleaned my briggs and stratton mower fuel tank and carb starts fine but " hunts" and as soon as i try mow it cuts out . When its running there is a spring loaded valve that keeps moving which drops the revs then goes up does this a few times before cutting out....any suggestions please....Andy

aseymour560 (author)2015-10-05


reyg2 (author)2015-09-05

The oil had water

Rmcmillon86 (author)2015-07-02

Could someone help me plz my husband tore down our yardman lawn mowers crankcase and after it was put back together the pull cord won't work can you please tell me what we did wrong and ASAP thank you

DirkJ1 (author)Rmcmillon862015-07-17

there are too many possibilities with that little information.

First, remove the spark plug and rotate the engine, does it rotate? Put a long screwdriver into the plug hole Rotate carefully making sure that the screwdriver does not get bound. As you rotate the screw driver should move up and down with the piston movement. If you can't turn the engine easily with the plug out then you have something inside that is wrong and binding the crank most likely. Time to open the pan again.

If that is all correct, then your problem might only be with the pull chord system. It is spring loaded and has pawls that move out when you pull the cord. They align into the top of the flywheel and force the flywheel to spin.

Take the pull cord assembly off the machine and look underneath...see how the pawls open and where they are designed to catch in the flywheel assembly. just examine it, You should see if there is a problem there. Assemble it correctly making sure the pawls are fully retracted at assembly. Then check it. Should be no problem then.

Kilo mike (author)2014-11-02

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

DirkJ1 (author)Kilo mike2015-07-17

This is usually the shear pin on the flywheel has sheared and the flywheel has spun. The flywheel is part of your magneto system and the magnets on the flywheel must create the spark at around top dead center. This pin aligns the timing of the engine. They cost about $1-3 and actually they do NOT hold the flywheel on at all. The flywheel is held onto a conical shaft by the torque pressure on the shaft nut, the shear pin is used for alignment only, like a disposable tool.

Make sure NO OIL of any kind (even skin oil) is on the shaft or inside the flywheel. The sheer pin should slide in easily. Then tighten the shaft nut to around 40 ft-lbs. (most engines anyway), you must hold the blades from rotating to do this.

Good luck.

nkhalsa (author)Kilo mike2015-04-23

I did something like that, and the problem turned out to be really insidious: The engine timing was thrown off so that the spark was occurring at the wrong time in the cycle.

DirkJ1 (author)2015-07-17

NEVER, EVER, EVER use a wire in a carburetor to clean any porting there. You can use nylon fishing line or similar products. Even copper wire can ruin the holes there, they are PRECISION machined and must not be damaged in any way.

It is best to use only spray cleaners and air unless there is some known blockage. An immersion tank with ultrasonic vibration and soapy water is the best thing to use. Then you can just rinse clean and perform a final cleaning with small amounts of carb cleaner and blow with dry compressed air.

HelsinkiCowboy (author)2015-06-27

I have a poulan pro 195 trimmer and it will not spark. I have changed the ignition coil and have gapped the magnito with a busniss card. The spark plug is gapped and I even used a second new sparkplug that was also gapped to ensure that the sparkplug was not to be the problem. The spark tester will spark but only a dull yellowish spark not a blue spark. And there is no spark at the plug. This is the second ignition coil as I believed the the first one that I purchased was faulity as it did not create a spark. Could anyone help me with this problem with suggestions - thanks.

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