Having a problem with my '00 WR400 which I'm hoping someone can advise on. It's fine when on full throttle, but on steady part throttle, or even accelerating gently (coming out of a slippery corner etc) it feels like it's missing - it's lumpy and juddery - doesn't pull cleanly at all. I've had it from new, and straight away put an aftermarket pipe on it with the bigger main jet that came with the pipe. A few months back the pipe was deemed too loud, so I've gone back to the standard silencer and the standard main jet. I thought it might be the pipe causing the problem, but I put the loud one on back today and the problem is still there. The only other mod I've made is to remove the airbox lid. Another thing that may be relevant is that around the same time I noticed the problem. I had another problem that it wouldn't rev out - just spluttered under full load. It was running a CR8E plug (the original!!) - the only spare I had was a CR9E and it worked fine with that. But when I put a new CR8E in I had the same problem on full throttle, so now I've stuck with the CR9E. Can anyone piece together the clues in this and have a shot as to what the problem might be?