ok it sounds like all that should be good then.
the screw on the mixer will not change the overall performance much, it mostly deals with the idle, once you accelerate past a certain point that part doesn't have much effect. If memory serves me correctly underneath where the fuel inlet is there is a coarse adjustment for fuel intake, sometimes that can make a difference but generally those are preset and do not need adjustment, but that one is the one that can make a difference in acceleration and overall performance.
What's it do when you remove the air filter? any change? (just curious)
Also since you said this engine is rebuilt? did you rebuild it? or is it pre-built and you just swapped it out? are you sure the timing marks set correctly? i would double check that.