Yz450f 2003 4th gear shudders under hard acceleration?

Anyone have an idea on cause and fix, also possible cost?

Exactly what 'shudders' ?

 

Are the bars 'wagging' back and forth?  That is 'headshake' and it's caused by suspenion sag and settings being off. 

 

4th gear wot, or old throttle?

 

Have you inspected your steering head, motor mounts, swingarm linkage, front axle pinch bolts, front wheel bearings, etc ?

Engine has a vibration in 4th under acceleration?

Would a 2005 gearbox be the same as I have access to one cheap?

Engine has a vibration in 4th under acceleration?

Would a 2005 gearbox be the same as I have access to one cheap?

 

It's not going to be the gearbox....

 

Again, your description is not specific enough...

Its got me at a loss? Steering head, rear linkage and wheel bearings have all been done?

Engine has a vibration in 4th under acceleration?

Would a 2005 gearbox be the same as I have access to one cheap?

 

Actually, it probably is the gearbox.  If your shudder occurs only in a certain gear, and only beyond a certain amount of throttle, and feels sort of as if the chain is slipping on the rear sprocket, you're skipping out of gear, so to speak.  The gears are engaged by means of blockish extensions and/or slots in their sides called locking lugs, or "dogs".  They start out with a very slight undercut so that they sort of hold to the other gear under a load.  Over time, the undercut wears over the opposite way, and the edges get rounded.  The gears under a load push each other apart instead of holding, but because they can't move the shift cam by pushing on the fork, the fork just flexes until the gear slips up off of one lug and drops into the next one. 

 

You'll need at minimum 3rd and 4th gears on the main shaft, and the forward fork, which will have become bent.  And yes, the '05 is exactly the same transmission. 

Thank you, this sounds exactly like what is happening. I think I'll grab the 05 Trans and pull her down and split her.

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