"Dave's Mods" refers to Dave Closs and Brian Jackson's carburetter modifications on an XR650L (they also apply to the NX650 which has the same carb and engine). Since the XRL and NX are street legal, they are jetted lean from the factory in order to meet emissions requirements. This makes the engines run a bit hot. Dave's Mods is designed to correct the lean condition and make the carb respond quicker when opening the throttle.
Click here for Dave's Mods:
My opinion on Dave's Mods:
Carburetor circuits are complicated and there tend to be a lot of misunderstandings about them. These guys know what they're talking about and I would not hesitate to use these mods, but when it comes to drilling the slide, you'd better be sure you're drilling the right holes (I think a new slide costs about $120). Some people claim big changes with these mods, some don't notice anything. I've tried these on one of my carbs and did not have a noticeable performance increase, but I live at nearly 9000' elev, so I have to jet my carbs lean anyway. Re-jetting will make more of a change than the rest of the mods. Despite my experience, I think Dave's Mods would make a noticeable difference at sea level with an free flowing exhaust system.