I have a 03 yz450f that I just rebuilt the whole thing after the previous owner airated the crankcase via connecting rod. Had cases repaired, replaced all tranny bearings, crank ect. Had to buy a different set of used trans gears as the big end bearng went through the old one when it scattered. The bike runs great shifts good ect. My issue is it seems to slip under hard acceleration in 4th gear. No issues in 1, 2 or 3. The clutch look pretty good to me when it was apart, nothing warped or burnt ect. Do I have a clutch problem or a 4th gear problem?? Any of the used parts I put in looked to be in decent shape. I have about 12 hrs on the fresh engine problem doesn't seem any worse or better. Don't want to tear it all down if I don't have to.