Why Does My Lawn Mower's Engine Speed Up & Slow Down?

Fuel

• If your lawn mower engine begins to speed up and slow down, the simplest solution of all, if you're very lucky, is that it needs gas. A lawn mower begins to cough as it reaches the bottom of the gas tank and will often speed up and slow down for several seconds --- maybe as long as a minute -- as it uses the last of the fuel. Before you do anything else, open the tank up and take a good look inside to make sure there is still enough there to run the engine.

Spark Plugs

• A lawn mower's spark plug is a critical step in the internal combustion process. A misfiring spark plug cannot supply the steady fire that keeps the engine's piston turning steadily. A common side effect of a fouled plug is engine racing and lugging. Like running low on fuel, replacing a spark plug is a simple matter that almost anyone can accomplish. The main thing to keep in mind is that the metal clamp that slides over the top of the plug is charged, so don't touch it with something metal like a screwdriver. Fingers are OK.

Filter

• Since a lawn mower needs a steady supply of clean air to run properly, the air filter becomes a likely culprit when it begins to run erratically. Once again, an air filter is exceedingly easy to change. It's normally located on the side of the engine in a rectangular compartment. Unscrew the screw holding the protective plate and remove the filter. If it looks dirty by visual inspection, try blowing or knocking it clean. If that doesn't work, it could be time for a replacement.

Carburetor

• Perhaps the most complex part of a small engine is the carburetor. If your lawn mower is running rough and these simple solutions don't improve performance, it might be time to consult a small engine repairman. The carburetor controls the flow of gas to the engine and a variety of associated issues could be to blame. Some shade tree mechanics have the necessary skills and tools to tackle a carburetor job, but if you have your doubts, take it to a professional.