More information please. What is the build? CV or Mik carb? How is the carb currently jetted? What symptoms make you think she's running rich?
The A/F screw is for idle and transition from idle to pilot/slow circuit. Depending on the motor configuration, exhaust, etc., that screw should be somewhere about 3 turns out, give or take. There is no fixed number of turns, every motor is different. When you bottom the A/F screw out, the motor should idle rough, stumble and ususally will die. If not, the pilot jet is too big. That's the first step to jetting and it needs to be done with the motor at operating temps.
The A/F screw has an o-ring at the tip; those can fall of/out and get lost; make sure the o-ring is in place. It may not come out of the threaded A/F screw bore as they will often stick in the bore.
Back the A/F screw out, slowly, like 1/2 turn at at time and give the motor a chance to adjust. Go out say 4 turns and back in 3 turns the same way. Listen to the exhaust and watch the tach and motor movement. At some point between 1 and 4 turns there will be a "happy" place, once you find it, note how many turns of bottom that happy place is and write it down. You are now ready to move to mid range tuning, i.e., pilot and needle.