Jet Needle

The jet needle is a long, tapered metal needle fixed to the throttle slide of a carburettor. It sits inside the needle jet, and as the throttle opens and the slide rises, the thinner part of the needle is pulled out, enlarging the gap and letting more fuel through.

The needle mainly controls the mixture from about a quarter to three-quarters throttle, the range used most in everyday riding. Its shape is defined by the straight diameter and taper, and the clip position in one of the grooves at the top raises or lowers it.

Raising the needle (moving the clip down) richens the midrange; lowering it leans it out. Needles and needle jets wear over time, causing a rich mixture and poor fuel economy, so replace them as a pair.