Popping out of 4th gear

[Gage.record15]

When the bike is accelerated in fourth gear it pops back to third, if you skip fourth and go to fifth the bike continues to run fine under acceleration, any ideas of what this could be? Thanks

[Foxx4Beaver]

Sounds like a bad circlip (broken).
There's a very slight chance it could be the gear shift mechanism spring, but probably not if it's only doing it in 4th.  Unfortunately, you'll have to split the cases, and don't just replace the one circlip, replace all of them....on a positive note, it'll be a great time to freshen up some bearings and anything else that might appear questionable.

[Gage.record15]

Thank you for the quick reply, I have the cases split but didn稚 know where to go next, I知 going to make sure everything is there according to the book and come back with pictures of anything that might appear worn or damaged, I expected there to be noticeable damage but I致e learned the transmission is very precise.

[sandblaster]

https://www.youtube.com/watch?v=2hLNLkojUMY

[Gage.record15]

Thanks for the video, everything seemed to work just as yours did in the video, I disassembled the two shafts and they have all of their parts and no damaged circlips. I noticed a few things while disassembling though, one being a few gear dogs that seemed somewhat worn, the second being that third gear was stuck on the countershaft, it spun freely but got caught when pulling it off, the splines are slightly burred where the gear sits. I have quite a few pictures of anything else you may need to see to try and help diagnose this problem.

[Gage.record15]

The wear report on the bottom end:

Shift Forks:






Countershaft where 3rd gear got caught up:



Countershaft 1st gear:


Countershaft 3rd gear:


Mainshaft 3rd gear dogs:



Mainshaft 4th gear:


Countershaft 4th gear dogs:



Could any of this be causing my popping out of 4th gear under load issue? What of this should be replaced so im not back splitting my cases again in a few rides? thanks

[KXDINO]

the dogs on 4th gear main shaft are rounded , repair or replace . while you at polish any part that contacts or slides on another part it will improve the shifting .

[sandblaster]

the dogs on 4th gear main shaft are rounded , repair or replace . while you at polish any part that contacts or slides on another part it will improve the shifting .

Agreed..
I would also at a minimum polish that shaft... It would be better to replace it.
Also, the third gear that rides on that shaft should have the ID checked thoroughly...
Compare to other gears that did not gall the shaft...

[Gage.record15]

I知 going to try to find a new shaft or one in better condition and a new 4th gear for sure, just a few more questions, is there anything else that looks like it should be replaced while I知 in the bottom end, Are the grooves in the one shift fork going to be a problem, and if I replace a gear is it mandatory that I replace the gear it mates with?

[Gage.record15]

Transmission parts are really hard to find so I think I知 going to take the approach of trying to freshen the gears up, do you guys have a place you could recommend that I can send this bottom end to to get freshened up and or what is your method to freshening the gears up?

[KXDINO]

Replace parts that are worn if there avaliable . i have seen them grooves before in shift forks with no problem , just spend time polishing and replace bearings seeing you come this far .you can still get output shaft new not the other shaft or the shift forks. someone is working on replacement shift forks , i cant thank them guys enough . i would replace crank seals and main bearings . on another note , there is a guy machining up new power valves for kx 2 strokes , he doing a lot of different models which is excellent , saw it on facebook.

[KXDINO]

There is methods to machine the dogs to get rid of the rounded section , and it works if done right.

[sandblaster]

Transmission parts are really hard to find so I think I知 going to take the approach of trying to freshen the gears up, do you guys have a place you could recommend that I can send this bottom end to to get freshened up and or what is your method to freshening the gears up?

Tom Morgan Racing TMR can fix your trans like new.
Or Steve Wells kx500tech is making new gears and shift forks..

[Gage.record15]

Thank you guys again for all the info, I知 going to try to contact Tom Morgan racing TMR to see if they can freshen this bottom end up for me.

[alward25]

send it all to TMR, and his answer will be yes he can.