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.
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
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
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
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
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
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.
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
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
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
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
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.