Check the gap on your spark plug with a spark plug gap gauge. Incorrect gap settings will lead to a carbon buildup because they do not fire correctly. 2. Clean or replace your lawnmower's air filter if necessary. Dirty air filters prevent the engine from getting the oxygen it needs to run well. 3. If necessary, clean the fuel filter on the mower to ensure the flow of gasoline to the engine is not restricted. 4. The carburetor of lawnmowers can get clogged by gas with 10% ethanol. Using ethanol-free gasoline in lawnmowers is the best choice.