Lean Mixture

An engine runs lean when the fuel-air mixture contains more air than ideal for the amount of fuel. Petrol engines need roughly 14.7 parts air to 1 part fuel by weight for complete combustion; noticeably more air than that is lean.

A slightly lean mixture improves economy and emissions at light load, which is why modern bikes are set lean from the factory. Too lean causes hesitation, popping on deceleration, hanging idle and high engine temperatures, and in severe cases detonation, burnt valves or a seized piston, especially on two-strokes.

Common causes are air leaks in intake rubbers, blocked jets, an aftermarket exhaust or filter without matching jetting or fuel mapping, and a fuel supply problem. A white or very light grey spark plug tip is a typical sign.