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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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jap0188622 days 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'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.... :)

alberto.padro.288@gmail.com

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.

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.

Taykaimyesterday

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 mike5 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

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.

wonderful.. I really like it...

RoseH16 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.
<a href='http://www.rpmsmallenginelansdalepa.com/' >
http://www.rpmsmallenginelansdalepa.com/</a>

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.... :)

alberto.padro.288@gmail.com

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.

www.bsmmrepair.com

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.
http://www.medwayimport.com/Service.aspx

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/

BruceA18 months ago

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

B.

Jjavaheri10 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
AWESOME
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: http://www.ciebookstore.com/small-engine-repair-course

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?
diva1912 years 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!!!
bdaoud4 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.
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