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Why does my engine backfire out the muffler?

Why does my engine backfire out the muffler?

"Backfiring" is a condition described as a loud bang, poof, explosion, etc. when idling the engine down, or after the engine has been shut off.

(Note: "Afterfire" and/or "Backfire" will not harm the engine at all!)


"Afterfire" occurs after the engine has been shut off.


Some possible causes include:

  • Shutting off the engine at high RPM. This can result in some fuel being pumped through the engine and into the hot muffler, causing that fuel to ignite.

  • Gasoline that contains alcohol has a tendency to ignite easier, plus cause the engine to run slightly hotter. This, combined with a hot muffler, can cause afterfire.

  • The type of muffler that has been installed by the equipment manufacturer may cause afterfire.

  • Carburetor adjustments may not be properly set to insure correct engine performance.

  • If anti-afterfire solenoid is installed it may not be working properly.


Some possible cures include:

  • Idling the engine down to the proper idle speed for 15-30 seconds. This would allow the engine to cool off some before shutting the engine down

  • Changing to a different non-alcohol or alcohol brand fuel

  • Adjusting the carburetor for optimum performance

  • Contacting the equipment manufacturer for updated designs in air control baffling, mufflers, etc.

  • Checking the anti-afterfire solenoid for proper operation

  • If equipped with an anti-afterfire solenoid, shut engine off at full throttle.
    NOTE: This after fire solenoid is specified by the equipment manufacturer


Backfire "typically” occurs when the engine is decelerated rapidly.


Some possible causes include:

  • Lowering engine speed too fast

  • Gasoline, which contains higher blends of alcohol

  • Carburetor adjustments 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. This condition cannot be corrected


Some possible cures include:

  • Lower engine speed slowly

  • Change brands of fuel to lower or non-alcohol

  • Adjust carburetor for optimum performance

  • Inquire with equipment manufacturer about increasing engine-cooling air volume by decreasing air restrictions.

In any event, your best source of information for any engine concern would be a Briggs & Stratton Authorized Dealer.


NOTE: Please read and abide by any applicable Safety Information (PDF) before performing any engine work. This information is not meant to take the place of work performed by a Briggs & Stratton Authorized Dealer. Terms and Conditions apply to all of our information provided on this website. Always be sure to read and understand your engine Operator's Manual.

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