I ride with my kids, and now that they are a little older we all rip around pretty good. But, when we ride in the woods by my house we are just creeping through. My 97 250R took a little time to jet right, but it has not fouled in 2 yrs. My 16yr old and I just jetted his new 98 YZ125 and it runns great, no fouling whatsoever. Click on the jetting links and it all makes sense and is pretty easy to do once you understand what jets control what rpm's and road speeds.
Also, we added flywheel weights to tone down the hit down low. I added a steel clutch which also added mass to the flywheel and made it rideable when picking through the slow rough spots.