What does a fuel screw do? How does it effect the fuel air ratio? At what RPM's and throttle openings? How do you know when to fiddle with it? Why aren't they stock? Sorry for the questions, but everyone seems to have them and say they are necessary along with the JD kit, but all i have done to my bike is chuck a 170 Main in and it has run fine with the standard uncorked exhaust or the PMB insert. I'm with ya on this one Mousemeat. I wish I woulda just left my bike stock, it ran fine before I started messing with the needle and jets. I just put in a NCVQ needle and the bike runs so crappy I wisn I had my XR 650 back!