What Causes a Lawn Mower to Smoke?

Oil Leak

• An oil leak can cause thick, white smoke. This is especially true if oil is leaking from the engine onto the hot exhaust. Check your oil level and see if it has substantially dropped after a few uses. Smoke from oil will be thick white or black and have a rubbery smell as it disperses.

Tilted Mower

• If you tilt a lawn mower in the wrong direction, meaning side to side rather than front to back, you can have oil flow in to the combustion chamber. If enough gets in there, you may not be able to start the mower. When there are smaller amounts present, you will have white smoke until the oil in the combustion chamber is burned off. This may be several uses, or longer.

Crankcase Leak

• A leak in the crankcase can cause gas and oil to mix. This will lead to thick white smoke and, if you hold a piece of white paper in front of the exhaust while the mower is running, you will see splatters of liquid on the paper, which is gasoline. A cracked crankcase or bad gasket requires an engine rebuild, which may cost more than a new mower.

Flooded Engine

• If you flood the engine, or if the carburetor is malfunctioning, you will get thick black smoke. The exhaust will also have a gasoline smell. Engine flooding can occur if you have a faulty spark plug and it takes several tries to start the engine. It can also be due to the user depressing the fuel bulb too many times.