There are two kinds of Mikuni TM carburetors. One type meant for two strokes, and another kind meant for four strokes. You've got a two-stroke one. It looks like different companies sell both for the XR600. Maybe that's the reason people have so much trouble tuning them. For the XR400, every company I've seen sells the four stroke version and people all love it (unless they care about fuel economy
Oh, and you might look into getting the frame powdercoated instead of painted. OregonCoast jsut got his SL frame and some other parts powdercoated by someone in Tillamook and they look fantastic.