4th gear issues 05 yz450?

Newish rider with bikes but eager mechanic..Probably something that's been posted a million times.. Anyways just picked up an '05 yz450 the first 3 gears engage flawlessly however as soon as it's dropped into forth and I hit the throttle it seems as if it's hesitating. The closest thing I could compare it to is a governor (but then again why would it be hitting almost as soon as it's placed into gear) limiting me from my top speed. Granted I know whilst riding most of the times I'm never going to be going full bore anyways. I'd still like to know if this an issue I should be concern about or not? Previous owner stated 10 hours on a top end rebuild to which to what I've been told is a long time. Any help would be great thanks in advanced. Feel free to ask any further questions for clarification.

A harsh jerking under a load?  The locking lugs on the 3rd and 4th gears on the main shaft are worn, and cannot hold in gear as they should any more.  The associated shift fork is also bent because of this.

Thanks for the reply! Should I be concerned about this affecting other gears whilst out riding? since I doubt that I'll be hitting 4th out on the trails. Just new to most of this and don't want to be sticking a band aide on a shotgun blast.

You should be concerned about the entire engine, since the whole assembly shares the same oil that 4th gear is dumping metal into. 

 

Look at the wear in these pictures.  It's what you will be looking for:

 

 

dog.jpg

fork.jpg

slot.jpg

Thank you very much gray!! I'll post pictures as it comes along!

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