the dellorto manual has very specific instructions for how to adjust your mixture screw. search and download that pdf and get reading.
the clifnotes version is get your bike to full running temp and put it up on the stand. turn up your idle to a little higher than normal, using the idle screw, then adjust the mixture screw in and out until you get the most even running position.
then, and correct me if im wrong, if you have to take it in to a position less than one full turn out, you need to get a smaller idle jet, and if you have to go to a position over 3 turns out you need a bigger idle jet.
ive heard different things about that last part, like its <.5 turn or >3.5 turns, but you get the idea.