Needle Characteristics
Thinner needles are overall richer, and thicker needles are overall leaner.
Needles with longer tapers are "earlier" because they begin to meter the air fuel mix at a lower throttle position, and they tend to be richer overall.
Needles with a shorter taper are "later" because they begin to meter the air fuel mix at a higher throttle position, and they tend to be leaner overall.
Needles with shallow tapers and fat tips are leaner, while needles with pronounced tapers and small tips are richer. As stated previously, a good place to start is with the needle clips. Raising the needle by putting the clip in a lower notch will richen the midrange mix, and lowering the needle by putting the clip on higher notches will lean it out. If you find the bike responds poorly on all notches or only runs okay on one of the very outside notches, you should look into getting a different needle.
Needle selection is somewhat of an art. One should choose a needle that delivers optimum mix at all its effective throttle positions. Unfortunately, this art is not a simple as going up or down a number like it is in jets. When tuning with needles, be very careful because an improperly tuned needle is just as dangerous as a main jet that is too lean --as I learned the hard way when I seized a Polini when experimenting by moving the clip from the perfect position to one notch leaner.
