Hey, I hope someone can help me. I bought a stock 99 WR400, with the exception of a ground off throttle stop, opened up exhaust and removed airbox lid, last month. I'v been out riding two times with no problems, until my third time out. It just started running real rough all of a sudden at about 1/4 to 1/2 throttle. I checked the air filter which was clean and plug which was really sooty, so I replaced it with NGKCR8E. That didn't change a thing. So I tore the carb off and cleaned it very thoroughly, put it back on and still have the same problem. While the carb was off I checked the jets and they are all stock and the clip was in the #3 position. It was running great before so I don't dare mess with the jetting until I trace the problem. What should I check now? Any input is greatly appreciated.