When a motorcycle backfires after being turned off, it is typically due to unburned fuel igniting in the exhaust system. This phenomenon is commonly known as an "afterfire." Several factors can contribute to afterfire:
Fuel mixture: An overly rich fuel mixture is a common cause of afterfire. When there is an excess of unburned fuel in the combustion chamber, it can enter the hot exhaust system. If the temperature is high enough, the unburned fuel can ignite, resulting in a backfire.
Exhaust system heat: When the engine and exhaust system are hot, the heat can cause the unburned fuel in the exhaust system to ignite. This can occur if the motorcycle was running at high RPMs before being turned off, leading to a buildup of heat in the exhaust.
Air injection system: Some motorcycles are equipped with an air injection system that injects fresh air into the exhaust system to aid in the combustion of unburned fuel. If the air injection system malfunctions or is not functioning properly, it can contribute to afterfire.
Exhaust leaks: Leaks in the exhaust system, such as cracks or loose connections, can introduce fresh air into the system. This can cause unburned fuel to ignite, resulting in afterfire.
Carburetor or fuel injection issues: Problems with the carburetor or fuel injection system, such as improper adjustment or clogged fuel injectors, can lead to an overly rich fuel mixture, increasing the likelihood of afterfire.
It's worth noting that occasional afterfire when turning off a motorcycle is generally considered normal. However, if it occurs frequently or is accompanied by other issues like poor performance or difficulty starting the motorcycle, it may indicate an underlying problem that should be addressed. If you're experiencing persistent afterfire, it's advisable to have your motorcycle inspected by a qualified mechanic to diagnose and resolve any potential issues.