Backfire

The term backfire was coined from early firearms in which the explosive force would be released out of the breach rather than the muzzle of a gun. The term backfire became associated with automobiles to refer to an explosion taking place in the exhaust system rather than the combustion chamber of a running internal combustion engine. Backfire in an internal combustion engine occur due to various malfunctions when the fuel air mixture timing is improper. 

Backfire can occur when an engine is running lean which means that an insufficient amount of fuel is delivered to a cylinder when the spark plug ignites the fuel/air mixture. This leads to an incomplete combustion which causes a misfire. In case of a misfire unburnt fuel is sent to the exhaust manifold (due to the spark plug firing a little later than it should have) where they may ignite unpredictably resulting in a loud popping sound. A misfire can also occur when an engine is running rich which means that excess amount of fuel is delivered to the cylinders. The excess fuel escapes to the exhaust manifold where they ignite unexpectedly. A backfire can also occur in the inlet manifold of an engine where the spark plug fires prematurely causing an explosion in the inlet manifold of the engine.

A visible flame may shoot out of the exhaust pipe when a misfire occurs in a large capacity engine which uses a considerable amount of fuel in the fuel/air mixture in a single cylinder.