AFAIK, the air screw affects mixture from idle to about 1/4 throttle. From there, it plays a lesser role. Needle jet kicks in from there till about 3/4 throttle, then maximum fuel delivery is determined by the main jet. But all three fuel circuits add together to affect mixture from that point up. The pilot jet can't be changed without access to the air screw, as mixture will almost certainly need to be tweaked.
It is an "air screw", not a "fuel screw" meaning that counterclockwise turning adds air, leaning mixture, while clockwise turning decreases air, richening mixture. if it starts well, but has difficulty idling when hot, try adding 1/4 turn of air. A tach is helpful in tuning and setting idle. Stihl makes one for chainsaw tuning that is a proximity tach. You hold it near or against the body of the scooter lower front near where your right foot would be. no need to physically attach it to the spark plug wire. Stihl edt5 is discontinued, but a very good one. Also, the Tech Tach tt-20K gets good reviews.
