Air-Fuel Ratio (AFR)

The air-fuel ratio (AFR) compares the mass of air to the mass of fuel in the mixture the engine burns. For petrol, about 14.7 parts air to 1 part fuel is the chemically ideal (stoichiometric) ratio, where all the fuel and oxygen can react.

A rich mixture (more fuel) gives a cooler-running engine and often more power at full throttle, but wastes fuel and fouls spark plugs. A lean mixture (less fuel) runs hotter, can cause hesitation, popping in the exhaust and, in extreme cases, a holed piston.

On carburettor bikes the ratio is set with jets and the needle; on fuel-injected bikes the ECU uses sensors and a lambda probe. After fitting an open exhaust or filter, the mixture usually needs adjusting. Spark plug colour gives a rough idea of how the engine is running.