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Backfire is when there is combustion anywhere outside the engine cylinders.

Backfire occurs after releasing the accelerator pedal suddenly on a carbureted engine. Some unburned fuel reaches the exhaust manifold and mixes with the fresh air injected. Sparks in the hot exhaust gas from the next cylinder that fires ignite the mixture. Backfire refers to the pop or bang heard through the air intake or exhaust system. It can be caused by an excessively rich or lean mixture. If the backfire is in the exhaust system during deceleration, the cause is a rich mixture. Lean mixtures usually cause a pop back in the carburetor.