Motorcycle Questions? Ask a Mechanic.
I can help you with that. The idle air mixture screw is the large brass screw on the top of the carb.
If it doesn't want to adjust properly then you likely have a damaged screw. It takes a very, very small variation in the taper on the end of the screw to not allow a proper adjustment. Also, if the screw is more than 2 turns out when that middle point is found then you probably have too large of jet sizes and need to work that out first.
I hope this helps, please let me know if I can be of further assistance. Thanks, P.J.