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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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
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.
SandraK49 days ago
shorty718114 days ago
My mower runs for about 15- 20 minutes then dies any help with this.? I have cleaned the magneto and the flywheel
mjuarezcna20 days ago
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?
ZidiP1 month ago
JerryW61 month ago
jap018863 months ago

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 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!
You could have a bent or worn crankshaft.

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.

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.

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>

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!

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.

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!

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.... :) need either an electric mower (where you have to negotiate mowing with a long lead) or an electric starting mower...or indeed someone else to mow for you. These arent machines to mess around with safety=wise.

The spark plug fires once for every 2 rotations of the crankshaft to ignite the fuel on the compression stroke of a 4 cycle engine. At 1000 rpm, the spark plug fire 8 times per second.

rspellicer2 months ago

My favorite solvent for cleaning carbs is lacquer thinner. The composition of lacquer thinner is very similar to commercial carb cleaners, but is cheaper. It won't leave a residue like gasoline often does. Of course, DO NOT USE ON PLASTIC PARTS.

Taykaim2 months ago

Great info here, although I have question regarding step 4.

If a crowbar is bad, what is the recommended way to hold the crank shaft in place?

Kilo mike8 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
nkhalsa Kilo mike2 months ago

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.

techlover20154 months ago

wonderful.. I really like it...

RoseH18 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.... :)

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/

BruceA110 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!


Jjavaheri1 year 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
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