Speed Demon

Member
Sep 23, 1999
3
0
I recently picked up an 05' YZ250F w/ some "issues".

The main issue is that the prevous own split the cases to rebuild the engine and have a broken part of the cases welded up. The only part on the outside that apprears to be damaged is on one of the bosses that the magneto cover bolts into by the shifter. It appears to be a very good weld job.

Problem: The bike goes into false neutral when trying to shift to 4th gear. Other than that it runs fine. The last bike I split the cases on was a two-stroke MX bike and that's been a while. Can anyone suggest what to look for and something to pay particular attention to? Is there anything else I should check before pulling the engine and tearing it down?
 

76GMC1500

Uhhh...
Oct 19, 2006
2,142
1
Bent shift forks, worn shift drum, worn/damaged engagement dogs on the gears. It will likely be obvious what the problem is once you get inside of the transmission. Check to make sure there are no missing shims as well.
 

SFO

LIFETIME SPONSOR
Feb 16, 2001
2,001
1
90% of the time, if you see a shiny slightly rounded engagement dog, you can replace that gear, the one the dog mates with, along with that specific shift fork, replace these items with new oem items, and given that everything is assembled as per your manual you will once again have a functioning transmission.
Read no further if rebuilding a stock transmission seems like a stretch.


What I have found is when gears get more room to travel axially along the gear shaft the ability for the gear to stay engaged on its dog gets reduced. Most good transmission shops (R&D transmission in Clearwater, FL) can sell you hardened shims for setting axial freeplay. I have also had to bend shift forks to get the gear to go exactly where I want it to go, not where the shift drum thinks it should go.
Check the bearing bosses on the sides of the cases where the gear can thrust against the case before sucking into the dog. Yamaha used to axially locate the thrust by using a circlip on the countershaft. I remember grinding that groove into new style c/s's to get positive axial clearance.
Transmissions are fun and very intuitive. Just follow the load as the transmission has to switch through its gears.
 

lwsmithjr

~SPONSOR~
Sep 18, 2002
194
0
When you say false neutral, can you ever actually get it in 4th gear? Or does it not pull at all in 4th and then you shift into 5th? Will it go to 5th?

I'm asking because my '03 YZ250F had 4th gear issues........I won't bore you with the details if your problem is different.
 

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