Troubleshooting lawn mowers
You only need to change the oil once every two years, if using modern car oils. Some lawnmowers don't handle synthetic oil very well. Some Briggs-and-Stratton mowers I've worked on will leak gas out of the air filter, for some reason, when using full-synthetic. On new Briggs-and-Stratton models, there's no drain plug underneath to change the oil; the sticker just says: "add oil when low." Just check the filler level and add if necessary.
The most important thing to have at the beginning of the year is starter fluid. In the event of difficult starting, spray some fluid into the air filter of the mower. Then, pull the starter cord. Keep doing this in addition to using fresh gasoline, until the mower starts.
Even after doing this step, there still remains several more problems related to the fuel system. Dirt can still clog the carburetor. Therefore, the installation of a fuel petcock (i.e. shutoff valve) is always advisable. Most people do not bother, but this could save you hours of needless hassle in the long run. All you have to do is turn the knob to 'close' position for winter storage. This way, no dirt will fall down into the carburetor during winter storage. Also, downstream of the fuel shutoff valve, you'll need a fuel filter for maximum protection! All these steps could be avoided if the lawnmower companies had just installed these two things at the factory. Why didn't they? Too cheap to. Modern cars have these things and much more, and they rarely have a problem with the fuel system (other than buying bad gasoline).
I haven't even gotten into the environmental aspects, yet. A fuel injector, while being very expensive to install on a lawnmower (and some do have them now), would save a decent amount of gasoline. They're easier to start and they make the mower more fuel efficient. There are many things that could be done to lawnmowers to make them environmentally friendlier and safer.
The next thing that will most likely go wrong is a loose or bad ignition module. This part is probably most responsible for abandoned lawnmowers, which is sad because I've restored many mowers that simply needed this cheap part. To fix the IM, you need a simple business card, a screwdriver, and a newly purchased IM (if necessary). Most times, the old IM works, but simply needs to be reseated. Loosen the screws and remove, readjust the old IM (if it still works), and wedge the business card between it and the flywheel. Remove the business card, and then tighten the screws to reseat the IM. Once properly gapped, the lawnmower should start right up!
If your lawnmower is overheating, make sure nothing is hindering the exhaust. On certain brands of riding lawnmowers, like the old Craftsman mowers, the hood can cover up the muffler, which causes heat to back up. The solution is to remove that section of the hood by cutting an opening.
If the riding mower overheats regularly, try cutting the grass in an irregular pattern. Make sure to cut the edge of the yard first, throwing the cut grass to the right. Then, circle around the yard and continue only throwing the grass to the right. This way, you are not throwing grass you already cut. This greatly reduces the load on the engine and helps to reduce heat. Also, throttle back a little. Don't leave the throttle at maximum the whole time while cutting.
As the riding mower ages, you might have a belt come off on occasion. You can still run the mower without having to change the belt, but you have to keep at the same gear speed. It might still fall off, but the chances will be less if you keep the deck height and speed constant.
The final thing you might encounter will be an older mower that has trouble starting. Any number of things could be the cause: bad sparkplug that needs replaced, a clogged crankcase breather, an over-or-undergapped spark plug, or a compression problem. The bad spark plug issue is pretty self-explanatory and requires only a socket wrench of the appropriate size. Crankcase breather problems, obviously, will need to be replaced. But in the case of compression problems, I say, best thing to do is just scrap the mower. Go get yourself a nice electric lawnmower! Most likely, you've got either a cracked intake or exhaust valve. A way to check for this is to, using a screwdriver, richen up the air-to-gas mixture ratio on the carburetor by turning the screw on the carburetor. This will allow more fuel into the combustion mixture. If it runs on a rich mixture ratio, then you've got compression issues. If the mower stalls in only moderately tall grass, this confirms compression problems. One final thing to check for is gas in your oil. This is if you see rainbow-colored oil when you drain the oil into a pan. That means gasoline is seeping around the piston rings. This means your carburetor is clogged and needs a good cleaning. What is happening is, the carburetor can't adjust the fuel ratio to meet the engine demands, because dirt is clogging the moving parts that allow for fuel ratio adjustment. One final thing: piston ring problems. This can also lead to a lack of compression. Again, I doubt if you want to pay to have the entire engine torn apart to fix this! Scrap the mower and get yourself a nice electric model!