It's the screw head that is on the side of the carb, right below the throttle linkage connecting point.
Turn it in 1/8th a turn a time, run it around for 2-3 minutes before you adjust it again.
The low speed needle (LSN) adjusts your fuel/air mix from idle to about 1/4 throttle.
The high speed needle (HSN) adjust overall fuel flow but is generally adjusted when you want to adjust performance/temps from 1/3-wide open throttle (WOT).
If the HSN is screwed all the way in, it will not allow fuel into the carb. If the HSN is adjusted, it may affect the LSN, so it would need adjusted as well.