G'day and thank you for your time. I have had this problem for a while now but I cannot pinpoint when it started happening or whether it always did. The problem is when you crack the throttle open from low speed/almost idle to over half throttle the engine chokes/hesitates until you ease back on the throttle. If you hold the throttle at this point for long enough the engine stops. It is only noticable in 2nd and 3rd gears. (just when you want to get the front wheel over a log etc.)There is no problem if you only wind the throttle on to 1/4 initially get past the 'hole' in acceleration then wind it on to full throttle. (You can still get the front up very easily and get wheelspin etc. at less than 1/2 throttle) This is what I have tried so far.(99 wr400, wr timing, Staintune exhaust with and without bung in, Twin Air filter, sea level) 1. 172 main, 48 pilot, 1.5 pilotscrew, dxm #4. I played with the needle with the best results at #7 but still hesitates slightly and used lots of fuel plus ran rich. At #1 it backfired alot and still hesitated. Pilot screw to 2 turns but no difference. 2. Went back to stock main 168, 48 pilot, 1.5 screw, needle at #4. Still the same problem. Does anyone have any suggestions on what may be the problem and how I can rectify it? Thank you. Jim Bob.