Riding Mowers & Tractors: Why does my riding mower run rough or misfire?

The carburetor mixes gas with air before the fuel goes into the cylinder. If the gas/air mix isn't balanced, the engine won't start or the engine will run rough and stall when you try to drive the riding mower. A damaged carburetor may also leak gas. Replace the carburetor if it causes any of these symptoms.

The carburetor mixes fuel with air to create a combustible mixture that enters the engine cylinder and ignites to drive the piston. Impurities in gasoline can clog the carburetor, preventing the engine from starting. Carburetor clogs can also cause the engine to run rough and the engine may stall when you try to drive the riding mower. Seals and gaskets in the carburetor eventually wear out and can leak gas. Use a carburetor rebuild kit to refurbish a clogged or leaking carburetor.

The engine tune-up includes routine maintenance for a riding lawn mower or tractor. Engine performance suffers if you don't regularly tune up the engine. A poor-running engine may stall when you try to drive the mower. You may notice uneven cuts if the engine doesn't run smoothly when you're mowing grass. Routinely tune up the engine by replacing the engine oil, oil filter, air filter, fuel filter and spark plug. The tune-up also includes checking the ignition, carburetor, battery, throttle and choke controls, as well as cleaning the engine cooling fins.

The engine fuel filter screens impurities from the gasoline going to the carburetor. A clogged fuel filter won't allow gas to flow to the carburetor so the engine won't start. A damaged filter won't screen impurities from the gasoline, resulting in a clogged carburetor. Replace the engine fuel filter if it's damaged or clogged.

The spark plug gets electrical current from the ignition system and sparks to ignite the fuel mixture in the cylinder, which drives the piston. The spark plug won't spark if it's coated with carbon deposits or oil residue, or if there's a crack in the ceramic insulator. The engine won't start if the spark plug is bad. A 2-cylinder engine runs rough when one of the spark plugs goes bad. Replace the spark plug if it's damaged or coated with residue.

The flywheel key is a small metal rectangle that keeps the crankshaft and flywheel aligned when you tighten the flywheel nut. To protect more expensive components from damage, the flywheel key is designed to shear if the mower blade hits an object like a rock or tree stump hard enough to make the flywheel slip out of alignment with the crankshaft. If the flywheel shear slightly, the engine runs rough or misfires. If the flywheel key breaks or shears severely, your riding mower engine stops running and won't start. If the flywheel key breaks or shears, replace it.