The AFM has a turn screw, bottom front towards the coil side, that adjusts the air-fuel mixture. It is normally covered by a cap, but if your afm has been "screwed with" the cap will be gone.This procedure should be done on a CO analyzer to get proper results.
Chad, Not much adjustment available. sorry. The AFM adjustment described by Steve is only for idle mixture. If you are running rich you may have any number of problems that can cause this. If your fuel pressure is low your injectors can be dribbling fuel into the cylinders and not effectively burning. If your timing is off or your valves are misadjusted you may be sparking too late or early to burn effectively. You could have a TPS that is out of adjustment or worn out or your water temperature sensor could be bad.
Hows your vacuum and compression?
Unfortunately it could be many things.