2003 & 2004 yz450f won't run in 4th gear any thoughts?

Over the past year we have had two YZ450F's in the shop and they run great in 1st,2nd&3rd gear but the second you shift into  4th they start bucking like they have a broken gear. On the first one we took the tranny apart and everything looked and lined up great. So is there anyone out here that has had this problem and knows how to fix. 

Thank You

Again?  The "dogs", the locking lugs on the sides of the third and fourth pinions (on the main shaft) are worn, and the forward shift fork is bent.  The contact surfaces of the lugs are originally slightly undercut relative to each other so the force transmitted from the third pinion to the fourth holds the two gears together.  Once they wear past the point where the undercut becomes reversed, they start forcing themselves apart under power.  The pressure on the fork can't rotate the shift cam, but the fork will deflect far enough the lugs to run over the top of each other and then snap them back together when the lugs line up again, thus the jerking.

 

Has nothing to do with broken gear teeth, normally.

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