Although it is also an internal combustion engine that works in a similar fashion to an automobile engine, a lawnmower engine is much simpler — though often just as frustrating. One common problem is when it revs up and down. Simple troubleshooting should allow you to track down the source of the problem. If it's something besides the carburetor, you might be able to fix it yourself. Unless you have experience, tearing into a carburetor can be a major undertaking. Keep in mind that there are only three areas to concern yourself with when it comes to small engine problems: fire, fuel and air.
The first step in troubleshooting concerns fuel. There are two basic problems you can run into here. The first, obvious though it might be, is when the engine is simply low on gas. A lawnmower engine typically begins to rev up and slow down for several seconds as it draws the last drops of fuel in the tank. Also related to this topic, and sometimes a culprit with a rough-running mower, is fuel that has water in it. A fuel container can gather condensation inside while being stored, or certainly as a result of being left out in the rain.
The second troubleshooting scenario concerns fire, which means the spark plug. A fouled plug or one that is firing out of time can cause your lawnmower to run poorly. Since spark plugs are relatively inexpensive and easy to replace, a couple of bucks spent at your local parts store could solve the problem of revving up and down. Take care to properly ground yourself while replacing the plug, since the rubber cap that slides over the top is electrified inside.
Air is the third area of concern with a poorly running lawnmower. A small engine pulls air from the outside in through what is called an air filter, which is located inside a rectangular compartment normally attached to the side of the engine. These filters often become clogged with debris, causing them to function less effectively. You can try blowing out the filter with compressed air or knocking it against a hard surface. Lawnmower filters are not expensive, so it's not a bad idea to change them frequently.
A high percentage of lawnmower engine problems can be traced to the carburetor. To troubleshoot this area requires you to partially dismantle the engine. If you're comfortable doing this and have some experience, go right ahead. Otherwise, it might be better to employ the services of a small engine repair specialist. Lawnmower carburetors aren't as heavy-duty as their automobile counterparts and frequently need to be taken apart for inspection, adjustment and cleaning.