Lawnmower Backfires When Starting

The Reason

• A lawnmower backfires when fuel is lit within the mower but outside of the combustion chamber. The fuel is located in pockets within the engine and exhaust. When the spark plug sends a spark to ignite the fuel, any fuel within range of the electrical charge will ignite, causing tiny explosions. The exploding fuel seeks an outlet: If the fuel is in the exhaust, a bright flame and loud bang will occur; if the fuel is within the engine, you may see a flame and smoke from the housing, along with the loud bang and mower shudder. Most backfires will erupt from the exhaust.

The Problems

• Two common issues result in backfires: incorrect air-to-fuel ratios and unregulated timing between the initial spark and the intake valves. Both allow fuel to escape into areas where it can cause damage when ignited. An imbalanced air-to-fuel ratio indicates too much air in the combustion chamber. The increased airflow lowers fuel pressure, causing an inability to ignite the fuel in time. Fuel, being liquid, seeks its own level and pools in the lowest areas of the mower, such as the exhaust system. Unregulated timing occurs when the intake valves fail to completely contain the fuel in the combustion chamber before the spark ignites the fuel. Lawnmowers are timed so that the valves close before the fuel ignites. Due to age and wear, the valves lose this timing and either fail to fully close or lose speed.

Troubleshooting

• Inspect the engine and carburetor for any gaps or holes. Look for metal fatigue, rust or areas of sudden impact. These openings allow excess airflow that reduces combustion and power within the engine. Check the fuel line for any cuts, gashes or holes; pay special attention to where the fuel line connects to either the carburetor or the engine. These gaps also allow excess air into the fuel flow. Look at the age and condition of the fuel pump and fuel filter. Dirty filters or pumps, along with age, can cause improper fuel flow. Check the spark plug, plug chamber and connecting wire for dirt, moisture or corrosion. These problems can cause weak spark, late spark or no spark at all.

Repairs

• Routine maintenance will catch most problems either as they occur or show potential problems. Maintain proper timing by cleaning the spark plug, wire and chamber regularly. Be aware of any sudden impacts with the lawnmower that may cause gashes, holes, ruptures or dents, increasing the airflow or allowing debris to enter the engine and carburetor. Replace the damaged parts before using the mower again. Change the spark plug and all filters annually to ensure their proper function.