
there are 4 slots on the needle, these are used to adjust mixture
by putting the circlip in a different slot, you're changing the position of the needle (gas) in relation to the slide (air)
1st (top) = leanest setting, not used by many
2nd = common setting for shipping & initial start-up
3rd = seems to be applicable to sea-level or high-flow setups, a pretty rich mixture
4th (bottom) = not used much, but some burn it fine at sea-level