I have a new GP3 fitted to my DB500 it runs ok on the bench , even ticks over a bit , but when out on the road theres a definite stutter at about 3.5k revs . If I close the air lever it is better so i feel that the needle should be lifted slightly i have a 350 main jet fitted . . At the moment its in the centre of 5 notches . So before I trial and error does this sound like the right way to go .
If you close the air lever it makes the mixture stronger, which lifting the needle also does (at least, in mid-range). So, you'd want to drop the needle, not lift it. However, needles came in two varieties to deal with that problem when running a megaphone. What exhaust system do you have, and which of the two needles is in your GP?