After finally getting my bike jetted right, after a 1hr ride my 00 WR400 started spluttering as if it was out of fuel. Switched over to reserve, no joy. Thus I thought dirty fuel in the carby. Only it lasted forever afterwards(20mins). The change was very much like a flick of a switch, appose to a gradual getting worse. In it's current state it basically idles fine, goes at full throttle fine, goes full throttle under load fine(thus maybe not a valve). Anything above 1/8 to 1/4throttle the bike splutters and farts as if it's running rich to running lean. Knowing this seems to be a needle position problem I moved the needle pos up 1 then 2up - no luck, then went the otherway - again no luck. The clincher is that it only starts to happen once the bike is warmed up to normal operating temperator. When cold to before normal operation temp - 1 to 3mins the bike will run 100% fine. What's happening here? I pulled the carby totally appart and cleaned it, went back to stock jetting etc settings, cleaned the spark plug which was black(checking the spark plug gap), all this - went well on the road till after 1min riding and then started actting up again... Noted that my previous jetting must have been too rich, but do I really need to purchase a new sparkplug to prove it is or isn't a fouling sparkplug type of problem.