Not sure exactly how to do it on that carb, but changing the jets and air bleed sizes will lean the carb out.

I would suspect if it is a used and dirty carb the air bleeds are clogged and it is "sucking" in too much fuel. They will be small (less then 0.01") holes which can been seen in the throat of the carb (at least on 4bbl holleys).

start there or do a complete rebuild.