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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

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.
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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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Rmcmillon861 month ago
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

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 mike10 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

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 Kilo mike4 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.

DirkJ11 month ago

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.

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.
SandraK42 months ago
shorty71812 months ago
My mower runs for about 15- 20 minutes then dies any help with this.? I have cleaned the magneto and the flywheel
mjuarezcna2 months 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?
ZidiP3 months ago
JerryW63 months ago
jap018865 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.

mamak_2 jap018863 months ago
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 starts..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.... :)

alberto.padro.288@gmail.com

alberto....you 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.

rspellicer4 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.

Taykaim4 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?

techlover20156 months ago

wonderful.. I really like it...

RoseH110 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/

BruceA11 year 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.

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!!!!

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