I am thinking about boring the carb on my 250XC. partly just for a project/experiment and also because some more top end would be nice. I am an experience machinist, but have never experienced boring a carb. The way i see it there are a few ways its done. Using my 36mm PWK as an example. oval bore- boring a 36mm hole 2mm above centerline, making a 36mm by 38mm oval. Straight out boring it to 38mm, but 2mm above centerline so that the slide floor is the same. Keyhole boring- boring 3 or more differnt arcs to create more volume above half throttle than an oval bore. Since there are those air fins, and the shroud cast into where the needle goes through the bottom of the carb I wont be able to go straight through. I imagine I will have to use a CNC or rotary table (Not going to use a dremel!) to cut a half arc on the bell side of the slide, in order to match the engine side. The other option I can see is to taper bore the carb so that it remains 36mm on the middle, but opens up to 38mm at the opening. This doesn't seem like it would provide very much gain though. Oval boring or keyhole boring sound like the best to me. I really like how the bike runs at low throttle openings, so I dont want to mess with the carb size at under half throttle. At least not yet. Am I way off in any of my thinking?? Thanks for any input!