Effects of Too Rich or Too Lean Air-Fuel Mixture on Motorcycle Engines

[OGrant]

1. When the air-fuel mixture is too rich, incomplete combustion occurs, generating large amounts of carbon monoxide and free carbon particles in the cylinder. This leads to carbon deposits on the cylinder head, piston crown, valves, and spark plugs, accompanied by blue smoke from the exhaust pipe and severe exhaust pollution. Moreover, the carbon monoxide in the exhaust gas may reignite in the exhaust pipe due to high-temperature exhaust gases, causing "backfiring" in the exhaust system. Additionally, the combustion speed of such a mixture is relatively low, resulting in reduced effective power and significantly increased fuel consumption. Furthermore, an overly rich mixture can wash away and damage the lubricating oil film in the cylinder, compromising the lubrication performance of friction components. A rich mixture also significantly impacts the engine's idle performance, manifesting as extremely unstable idle speed, severe cases of no idle, and ultimately causing the engine to stall. In such cases, removing the spark plug will reveal blackened electrodes, confirming an overly rich air-fuel mixture. 2. When the air-fuel mixture is too lean, the combustion speed of the fuel decreases. Some of the mixture may continue burning as the piston moves toward bottom dead center (in four-stroke engines, where the combustion chamber volume gradually increases). In severe cases, the combustion process may even extend into the intake stroke of the next cycle, causing residual flames in the cylinder to ignite the mixture in the intake pipe and carburetor outlet chamber through the open intake valve. This results in backfiring (also known as carburetor backfire) and produces knocking noises. This partial combustion not only converts less heat into mechanical work but also increases heat loss through the cylinder wall to the cooling water or radiator fins. Consequently, an overly lean mixture causes engine overheating and reduced power output. A lean mixture also leads to higher idle speeds, making it difficult for the engine speed to drop to idle quickly when decelerating. In such cases, removing the spark plug will show white electrodes, confirming an overly lean air-fuel mixture.