How can I make my 750 Quick Fuel run leaner

With the mixture screws only out 3/4 and everything else seeming ok, you may be at the point of opening up the idle air bleeds a little......

http://www.quickfueltechnology.com/technical-info/carb-class-air-bleed-basics

P.S. Since these bleeds are so small, i should also recommend hitting them with a blast of carb cleaner first just incase there's a little dirt buildup.