mixture screw adjusts amount of fuel when on the intake side, amount of air when on the air filter side of the carb body. On PHBG it is on the intake side so adjusts amount of fuel. clockwise will lean your mix, counter clockwise will enrich our mix. I find that the leaner the better (providing it will still idle) in terms of throttle response from take off. PHBG manual indicates that you should be sitting somewhere around 1 and one half turns out from all the way in (if I remember correctly). once you get within a turn or less out from all in, it is time to step down to a leaner idle jet. Currently I am running a #50 idle on my AV10 race bike -- probably a #45 would be better as I am only a half turn out.