Freaky, I came here looking for information on this very topic. I too am fouling plugs on my '03 yz250. If I can make it through the warm up, it's usually pretty good (stone stock on every part, as delivered by Yamaha, with the exception of an FMF gnarly pipe). Usually starts without choke on the 3rd kick when cold. I ride at about 2200-2400 feet of elevation. Running 91 octane pump gas with Yamalube "R" mixed 32:1
I would say that evil exhaust pipe spooge is at a minimum for me. just a little bit around the tip of the pipe after all summer running.
HOWEVER, Yesterday I fouled 2 plugs, one on start up, and one about 3 hours into the ride. Both were new plugs, fouled the 9, stuck in an 8, fouled it on the trail 3 hours later.
Here's the question. The plug I fouled on the trail I have a pretty good feel for the exact scenario that lead to it. I was on some twisty little 2nd gear stuff, and I had been holding the throttle about 1/4 for about 3-4 minutes in 2nd, not bogging, not in the powerband, not revving, just happily motoring the engine in a good spot on a flat twisty trail. I got to the end of the trail, had a heckuva hill to climb to get out of the trail, and it sputtered to the top and promptly fouled 5 seconds later when I was on the pipe trying to get up this hill. What needs to be changed? I thought the needle was mostly a top end adjustment. My top end feels fine, the bike is crisp all through the power band, and I have no complaint at all (short of the fouling plug thing). What should I adjust?????????
I can't help but think a few more minutes at the lower RPM would have wiped out the plug anyway, but the hill finished it off first. I'm perplexed. Also, both of yesterdays fouled plugs looked good in color, but were soaking wet. (they should look good, they were both new, need to buy plugs in bulk).
Thanks,
-Bob