What Are the Causes of a Lawn Tractor Backfire Through the Exhaust?

Overall Cause

• Lawn tractor engines use small explosions within the combustion chamber to generate their power. When an explosion occurs outside the chamber a sound similar to a gunshot and a sudden quick flame erupt; this effect is called a "backfire". Most backfires occur through the exhaust. Burning fuel entering the engine or exhaust system causes the backfire. Unspent fuel ignites as a spark occurs close to the pocket of fuel that settled in the exhaust or engine. These pockets form in low spots in the exhaust.

Fuel Issues

• Check the fuel pressure of the lawn tractor. Low fuel pressure prevents an engine from generating the power required to ignite fuel for the needed time interval. The result is unused fuel forming pools in low spots in the exhaust. When too much air is allowed in the combustion chamber it creates an imbalance in the air-to-fuel ratio, thus creating the low fuel pressure. Examine the fuel pump or fuel filters to determine the cause of the problem; failing fuel pumps or old and dirty filters are the main cause.

Engine Issues

• Determine if there is a timing problem with the engine. Faulty timing causes sparks to light the fuel before the intake valves are closed. When intake valves are not fully closed, fuel flows into the engine where it ignites, causing a backfire. The timing between the ignition and valves is regulated when mowers are properly maintained. Over time, this timing becomes unregulated as the spark misfires. Examine the spark plug, plug wire and plug chamber for dirt and debris. Examine the fuel filter for dirt, debris and aging.

Solutions

• Routine maintenance will catch most problems as they occur or it will show potential problems emerging. Maintain proper timing by cleaning the spark plug, wire and chamber regularly. Look over the engine and carburetor for gaps or holes due to metal fatigue or corrosion. Sudden impacts against the tractor may cause gashes, holes, ruptures or dents causing increased airflow or allowing debris to enter the engine and carburetor. Replace the damaged parts before continued operation. Change the spark plug and all filters annually to ensure proper use during their lifespan.