The air screw is just for the idle setting. Here is what my manual says:
Start and warm up the engine for five minutes.
1. Turn the throttle stop screw to the lowest idle speed
2. Turn the air screw in or out to find the highest idle speed
3. Set the idle speed by turning the throttle stop screw.
The idea here is to just get a dependable idle at a slow enough to not be engaging the centrifugal clutch.