Idle Mixture

The idle mixture is the ratio of air to fuel the engine receives at tickover and very small throttle openings. On a carburettor it is supplied mainly by the pilot (slow) jet and fine-tuned with the pilot air screw or pilot fuel screw.

A mixture that is too lean can cause hanging revs, popping on deceleration and hesitation when opening the throttle. Too rich a mixture gives a lazy, uneven idle, sooty plugs and a fuel smell. Altitude, temperature and air filter condition all affect it.

To adjust, warm the engine fully, then turn the screw slowly in small steps to find the highest, smoothest idle, and reset the idle speed afterwards. On fuel-injected bikes the ECU controls the idle mixture.