My son and I have both recently acquired 2007 XR650Ls. Nice bikes well maintained with low miles -- nearly perfect condition. I'm heavy at 270lbs and my son is probably around 250lbs. There is no reason to expect that the sprockets are not the original sizes. At 45MPH my bike feels unhappy in 5th gear. 4th smooths out the ride quite a bit. The bike doesn't like 5th gear until a little bit past 50MPH. Is this typical?