Been looking all over for an answer on this, with no luck. 2000 XR600r with an aftermarket exhaust: full pipe and FMF power core 4. Running Uni foam air filter. Have a Mikuni 41 mm flatside carb I purchased from XR's only. Driveable, but bike has been running rich since I've put the carb on. Currently installed is a 250 main and 15 pilot. Idle wise, seems ok, but on snap accel I get a light stumble, seems to be less common when the bike is warmer. At idle, when snapping open throttle to 3/4 quickly I see some black smoke. Also some surging at low speed steady throttle cruising. Recently dropped the slide needle to its lowest setting, which improved it a little, but still obviously rich. Wondering if I should go down to a #240 or #230 main instead. Seems most people are all running #250's with similar setups as mine. I am at 500 ft above sea level in Alaska. Any experience on this would be greatly appreciated.