Backfiring (or more accurately “afterfire”) is caused by unburned fuel/air mixture being ignited by the heat of the muffler. When the throttle is closed during de-acceleration and the transmission is in gear, the engine continues to rotate at a faster rpm relative to the throttle position.
As a result of this engine rotation, more air/fuel mix is pulled through the carburetor or injector throttle body (dependent upon it’s ECM program). This mixture can then build up in the hot muffler and ignite. The bang heard is the sound of this fuel igniting after the throttle has been closed while de-accelerating.
There are several other factors involved such as: ambient temperature, fuel additives, carburetor settings, ECU programs, alcohol enriched fuels, etc. that may also contribute to the “afterfire” condition.
Stock restrictive mufflers mask this condition. Aftermarket mufflers are more open and allow the “bang” to be more apparent. Some types of exhaust systems such as “True-Duals” on Harley-Davidsons seem to magnify the affect. Engines with carburetors can usually be re-jetted to minimize the “afterfire”. Minimizing the “afterfire” effect of fuel injected bikes is dependent on the ability to reprogram their ECU’s. By Khrome Werks, a division of Lincoln Industries®