I'm having about the same problem as Roger here. I ride a WR 400 -98 at about sea level. I have measured resistance at the ignition coil and plug cap and everything seems to be fine. The spark from the plug looks OK. The cams are on standard timing (I've checked of course). I have experimented alot with different plugs and the plug sometimes takes longer time to foul if I use a CR7E but it will foul eventually. In the summer they last longer but in the winter (-10 celcius) the plugs foul very quick. My fouled plugs are all black and sooty. I have experimented with different main jets (even extremes like 132 up to 190) but the plug will be black after like 10 minutes (wintertime) anyhow. In summer I can usually use the same plug for a couple of hours of riding.
I will disconnect the APJ according to the description above during the weekend and have a try with that. One thing that is a bit surprinsing is that I can turn the choke off after just a couple of seconds of running even if it's -10 celcius. Any ideas about that? Could it be way rich on the pilot jet?
The bike runs great until the plug fouls by the way. It doesn't smoke or anythink like that.
Will it be worth a try to fit a YZ 426 -01 carb on it by the way?
Any ideas would be greatly appreciated, the bike has been like this since I bought it (used) 2 years ago.