If fire is coming out of the exhaust of a motorcycle, it is usually due to unburned fuel igniting in the exhaust system. This phenomenon is often referred to as an "afterfire" or "backfire." There are a few possible reasons why this might occur:
Rich Air-Fuel Mixture: If the air-fuel mixture in the combustion chamber is too rich (excess fuel), it can result in unburned fuel passing through the engine and into the hot exhaust system. When the unburned fuel encounters oxygen and heat in the exhaust, it can ignite, causing a flame or fire to come out of the exhaust.
Ignition Timing: Incorrect ignition timing can lead to an afterfire. If the spark plug fires too early or too late in the combustion process, it can result in unburned fuel entering the exhaust and igniting.
Exhaust System Design: The design and configuration of the exhaust system can influence the occurrence of afterfires. Factors such as the length and diameter of the exhaust pipes, the presence of baffles or other restrictions, and the efficiency of the muffler can all affect the likelihood of afterfires.
Modified or Performance Enhancements: Aftermarket modifications or performance enhancements, such as aftermarket exhaust systems or fuel tuners, can alter the air-fuel mixture and ignition timing, potentially leading to afterfires if not properly calibrated.
It's worth noting that occasional afterfires may be considered normal in certain situations, particularly during hard deceleration or when the engine is cold. However, persistent or excessive afterfires can be an indication of an underlying issue with the fuel system, ignition system, or exhaust system. If you are experiencing frequent or significant afterfires, it is recommended to have your motorcycle inspected by a qualified mechanic to diagnose and address the problem.