2001 XR250 Jetting

Hello,
I'm not positive on the 250's, but I know that the 98+ 400's are jetted way leaner than the 96/97 models. They are good stock, but any airbox/pipe mods will increase airflow, thus leaning out the mixture. Maybe he could just adjust the airscrew to compensate, but rejetting is probably needed to fine tune the mixture. Put in a new plug, and pull it back out after a short time of riding. It should be a darker charcoal, or brownish color. Black and sooty means to rich, light grey or white means to lean. Good luck.

Get a Thumper insert,aftermarket filter,go up two on the pilot and main jets and RIDE.

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According to the Dirt Bike article in June 2001, the bike and jetting should fine with the air box stuffer and exhaust baffle removed. I'm running mine that way, but haven't had a chance to do a plug reading yet. Bike runs great though! I've run the thumper insert on a 400 and it IS a good way to keep the noise down. Mine stuck out kind of far...don't know if they are still that way. Make sure your friend's air filter is oiled well and properly seated too.