This saga is like the 'Neverending Story'. Maybe reading this thread infects your bike... But really, I've already posted that my '01 suffers the same problem of fouling plugs. Jetting changes have no effect. It's NOT the air filter. Yes, I'm familiar with the problem of trying to start the bike with a freshly-oiled filter but I've use thrououghly dried filters and the problem persists. These are the same filters I used on my '00 426 with no problems. I've been more observant about the bikes behavior and have noticed a few things: 1) the bike starts fine (new plug), idles great and initially runs cleanly, 2) after, say, 5-10 minutes of running (varies depending on temperature) the bike goes through a phase that involves missing/backfiring/popping and the idle speed drops so that it's necessary to constantly blip the throttle in turns, 3) after another ~10 minutes (if the plug doesn't foul) the bike starts to run better as if the mixture has leaned out to normal levels, 4) the bike will now run properly (or almost considering the almost-fouled plug) for the rest of the day - unless you stop and take a break, 5) after the break, upon restarting the bike it will go through the same period of poor running/popping/backfiring and poor idle and then clear up again (provided the plug still works). I thought that the answer might be the temperature/temperature distribution of the carb due to a misassembly/machining error but now, after reading about how a replacement carb started suffering the same problem after everything was fine for a while, I don't know. Also, I seem to remember that the first ride (during break in) didn't go through the poor running phase. But if it's not the carb then how did a replacement 'cure' the problem for time? This is getting very strange. In my garage I have my '00 426 which has always run fine. I'm wondering if I should try swapping carbs/cdis between bikes. I know there have been a number of changes including spark advance so I don't know if I'll learn anything.