I just got this 06 yz450 and im pulling out my hair. I was told it has a fresh top end, and it looks like a new head. so I live in South Lake Tahoe 6300 ft elv. so not having the opportunity to ride it when it was bought for me by girl friend. when i tryed to start it, it backfired 3 or 4 times before it reluctantly fired and ran very lean. So my first thought was valve timing or a shim was tweaked,, nope that wasn't it so i tried carb. found a disintegrating accelerator pump diaphragm I thought ah ha i found the problem, nope. So Im not sure where to go with it.