The air screw only affects the pilot circuit (idle to 1/4 throttle). From 1/4 to 3/4 throttle you're on the needle. From 3/4 on up you're on the main jet. The pilot and needle should be tuned for best throttle response. If you adjust the air screw and find that it's less than 3/4 turn from all the way in, you should install a richer (higher number) jet, beyond 2 1/2 go to a leaner (lower number) jet.
The main should be tuned for maximum power, but be carefull, there is a fine line between correctly jetted on the main and too lean. If you get the main too lean you run the risk of a ceased piston.
No one can tell you how to jet your bike, there are too many variables. If you think you are running rich, try raising the clip on needle. See how that affects throttle response in the 1/2 throttle range. Don't woory about getting the idle and needle circuits too lean, you really can't do any damage...at worse you'll get a lean bog.
Before you start to tune your carb, make sure you have done the following:
1. Cleaned the air filter;
2. Cleaned the silencer;
3. Installed a freah spark plug;
4. Addressed any other induction/exhaust problems.
Good luck and have fun.