Main jet screw doesn't adjust. It's just for access to the jet. Tighten it barely more than snug.
Idle screw adjust idle. In for higher idle, out for lower.
Air screw, or pilot screw, meters airflow. It's got a sharp pointy tip. Should be screwed in till it stops (lightly, so you don't damage the tip), then backed off 1.5-2 turns. That's the ideale setting if your carb is jetted right.
If the bike runs better with it screwed in all the way, your pilot jet is too big, if it runs better screwed out more than 3 turns, jet is too small.
Idle should rise as you unscrew it from seated, until it starts dropping. Then screw it back in a 1/4 turn or so and make sure your idle doesn't "hang," (meaning it stays high for a minute, then suddenly drips,) and that it doesn't hesitate when you blip the throttle.
If it does either, it's usually the wrong size jet.