Backfiring

A back-fire or backfire is combustion or an explosion produced by a running internal combustion engine that occurs in the air intake or exhaust system rather than inside the combustion chamber. Unburned fuel or hydrocarbons that are ignited in the exhaust system can produce loud sounds even if flames are not present at the tailpipe. A visible flame may momentarily shoot out of the exhaust pipe where the exhaust system is shortened. Fire may also travel into the air intake piping. Either condition may cause a loud popping noise, together with possible loss of power and forward motion. A back-fire is a separate phenomenon from the fire produced by Top Fuel dragsters.

If a backfire does occur in the exhaust, that is called an after-fire. The term backfire is when unburned fuel moves back into the intake, and combusts, whereas an after-fire combusts unburned fuel in the exhaust side of the combustion cycle. A likely cause of this is from running rich, which in certain cases could be from the combustion not achieving high enough temperatures to correctly burn the fuel. Meaning a foul spark plug, coil, or plug wire could be the cause. In most cases, after-fires are bad due to the combustion, or fire within the muffler or exhaust system. This will over time cause damage to the mufflers and piping of the exhaust—eventually leading to an exhaust leak, and a burned out muffler/catalytic converter. Also there will be performance lost due to a weaker combustion.

Also, an explosion in the inlet manifold, carburetor/throttle body, or air cleaner of an internal combustion engine can occur when the intake valves are not shut prior to fuel combustion.

The term derives from parallel experiences with early unreliable firearms or ammunition, in which the explosive force was directed out at the breech instead of the muzzle. From this came the use of the word "backfire" as a verb to indicate something that produces an unintended, unexpected, and undesired result.

...
Wikipedia