A lawnmower that is smoking usually indicates that either the fuel or oil compartments are overfilled, and the engine is burning off the excess. Blue smoke typically indicates that oil is burning and may be evidence of a carburetor problem. Black smoke indicates the carburetor is burning off gas.
A lawnmower that smokes during the first few seconds of starting up is usually considered a minor problem, while light smoking throughout accompanied by power shortages indicates a motor problem and should be serviced immediately. Two-stroke engines always produce a small amount of smoke when starting, while four-stroke engines should produce virtually no smoke at all.