Re: where is the air fuel mixture screw located on the carb?

With 2 1/2 turns instead of 1 1/2 you are going in the rich direction not lean.
The carb and throttle linkages on those stick often so your internal butterflys may
not be in position they should be.  It might be useful to remove and clean the carb, inspect the jets and operation of the linkages.  Look into the throat while hand moving them.