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.

What are engine backfire and afterfire?

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!

What causes a small engine backfire & how do I fix it?

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:

What causes a small engine afterfire & how do I fix it?

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.

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