Started having a problem with my son's 250 today. Has run flawlessly for 5 years...Took it out to the track today and at full throttle, the bike wanted to sputter, spit and backfire. If you feathered the throttle in any gear, the quad would eventually get up to speed, but you could never goose it without it suddenly losing power and backfiring. Is it time for a carb rebuild or something else more sinister? New Battery BTW if that matters.....