When your mower backfires, it can be loud and sometimes scary. Backfire in engines is caused by simple chemical malfunctions that can be easily rectified. A backfire in a lawnmower engine can damage both the engine and the exhaust as burning fuel explodes. Take care to avoid close proximity to a backfire and the motor until you've determined the cause and fixed the problem.
Terminology
Lawnmower engines are considered "small engines." Small engines use a series of small explosions in the combustion chamber to generate power. This power allows the motor to function. When one of these explosions occurs outside the combustion chamber, such as in the exhaust system, it creates a bang and possibly a sudden quick flame that burns out as quickly as it erupts. This "bang" is called a backfire.
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The Cause
Backfires occur when burning fuel enters the engine or the exhaust. If pockets of unspent fuel enter the engine before the valves close or escape to the exhaust system, a backfire occurs. Unspent fuel ignites when a spark occurs in close proximity to the fuel pocket.
Fuel-related
Determine the reason for low fuel pressure. Low pressure is the imbalance of the air-to-fuel ratio where there is too much air in the combustion chamber. The result is that the engine cannot create enough power to ignite the fuel for several seconds. The fuel then pools in low spots along the exhaust. Causes related to low fuel pressure include failing fuel pumps and old fuel filters.
Engine-related
Check the timing of the motor. If the timing is off, the engine can ignite a spark prior to the intake valves closing. This causes the fuel to ignite within the engine, causing a backfire. Over a period of time, the timing between the engine and the valves becomes unregulated and requires a tuneup. Changing the spark plug, cleaning the wires and plug chamber and changing the fuel filter annually usually completes tuneups.
Troubleshooting
Examine the engine, carburetor and fuel line regularly. Look for gaps and holes in the metal of the engine and carburetor. Rust, metal fatigue and severe impact against the mower can cause gaps and holes. These holes allow excess air and reduce engine combustion. Look for gashes, holes or cuts in the fuel line and where it connects to the engine and carburetor. This damage also allows excess air into the fuel and motor. Replace any damaged parts immediately to avoid backfire.