Clutch or gears

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.

Thanks. I think I have a trans problem. My clutch does have free play on it and it is a very fast rise in rpm then when I back off a just a little it goes bakc in. As I increase speed it get easier to make it do that. Would keeping pressure on the shifter while accelerating help diagnose the problem as in hold it in gear or feel some feed back on the shifter?

Would keeping pressure on the shifter while accelerating help diagnose the problem as in hold it in gear or feel some feed back on the shifter?
Maybe. But even if the gear pushes itself out of engagement against the fork, it still cannot cause the shift cam to rotate as the fork cannot drive the cam. The feedback would be somewhat indirect. If you absolutely cannot make it do this in any gear but one in particular, that's another sign.

Clutch slip is more like a tire slipping in poor traction, except it happens more in the higher gears than the lower ones.

Clutch slip is more like a tire slipping in poor traction, except it happens more in the higher gears than the lower ones.

Thanks man. I think I will be splitting cases. Mine problem is a for sure on and off. I revs up and re-hooks almost instantaneously like an on off switch.

Gears and forks for sure then? If I remember my gears looked not to bad when it was apart this winter. Or is doing both just a good way to be sure?

If the gears separate under a load, the fork will be bent.

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