Why Does My Engine Backfire or Afterfire?

Is the small engine on your lawn mower, snow blower or outdoor power equipment making loud noises? This FAQ from Briggs & Stratton will help you identify the problem and find solutions.

Backfire is a condition described as a loud bang, poof, explosion, etc., while the engine is running or while shutting down. Afterfire occurs after the engine has been shut off. Note: Backfire and afterfire through the carburetor will not harm the engine at all!

Backfire typically occurs when the engine is decelerated rapidly.

Common Causes of Backfire:

• Lowering engine speed too fast
• Gasoline, which contains higher blends of alcohol
• Carburetor adjustment set too lean
• Muffler construction can induce backfire
• Higher than normal engine temperatures
• Some carburetors can induce backfire due to the sensitivity of internal transitional passages
  Note: This condition cannot be corrected

Possible Fixes for Backfiring:

• Lower engine speed slowly
• Follow small engine fuel recommendations and/or switch to brands with low or no alcohol
• Adjust carburetor for optimum performance

Afterfire occurs after the engine has been shut off.

Common Causes of Afterfire:

• Shutting off the engine at high RPM, causing fuel to pump through the engine for ignition
• If equipped with an anti-afterfire solenoid, shut engine off at full throttle
• Small engine muffler type and manufacture
• Carburetor adjustment may not be properly set for correct engine performance
• Anti-afterfire solenoid may not be working properly

Possible Fixes for Afterfire:

• Allow the engine to cool by idling the engine down with proper speed (15-30 seconds)
• Change to a different non-alcohol or alcohol brand fuel
• Ensure proper carburetor adjustment for optimal engine performance
• Contact the equipment manufacturer for updated designs in air control baffling, mufflers, etc.
• Check the anti-afterfire solenoid for proper operation
  Note: If equipped with an anti-afterfire solenoid, shut engine off at full throttle

In any event, your best source of information would be to consult your local Briggs & Stratton service dealer.