When a motorcycle engine has an emission system malfunction, such as an exhaust leak or a moment of running rich or running lean, backfire occurs as a result. When an engine is running rich, there is more fuel present than air, according to DoItYourself.com. When it is running lean, there is more air than fuel. In either case, the result is an incomplete combustion causing the loud popping noise.
A backfire is an explosion that occurs either in the intake or in the exhaust of the motorcycle. This explosion, however, should occur in the combustion chamber. When it doesn't, there is an interruption in the engine's operation. It is possible to see a short-lived flame coming from the exhaust. Whether it happens in the intake or the exhaust, the loud noise that results can be momentarily startling to the rider and those nearby.
The stock muffler that comes on a motorcycle is designed specifically for the complete and proper operation of the engine. When an exhaust pipe is changed, this causes an unbalanced air-to-fuel ratio. The airflow into the motorcycle is increased because of the difference in pipe design and other specifics, and this difference causes backfire in the exhaust. Other causes of backfire are bad or weak fuel pumps, low fuel pressure and clogged fuel filters. All three of these issues have a direct effect on the air-to-fuel ratio.
There are several ways to prevent backfires. The carburetor must be clean, as fuel cannot flow properly through a dirty carburetor. If it is dirty, it needs cleaning with a high-grade carburetor cleaner. The jets must be unclogged, because jets clogged with debris can also prevent fuel from getting through the engine properly. If the motorcycle is fuel-injected, a high-grade cleaner can remove dirt and debris trapped in the fuel line. Finally, a higher grade of fuel is in order to clear fuel lines and keep the fuel tank clean.