This is a mod Honda FYI. I went road racing and didnt change much to convert from a sprint setup other then the gears. As the practice sessions went on I i made one change to the axle gear and ended up going back to where I started. I started at 18/23 and tried 18/24 and ended up going back to where I started. After that I noticed 5th gear being kind of jumpy and not holding. I split the case today to see if there was anything noticeable. The fluid was clean with no chunks of metal or metallic flake in it. The gear stack appeared to look brand new with very very little wear on the cogs or dogs or whatever they are called. I did notice the axle gear had a bit more wear on it as if it was not perfectly aligned with the drive gear. But would this just affect 5th gear? Here is a video of what was happening.
On a side note I am currently using an RS125 piston and Im having a hard time wither finding one and I cant find one that is a reasonable price. Is there another piston thats a close 2nd to this piston? I was looking at the vertex piston kit.
I just went through this with my kart. I was getting a false neutral in 5th gear and the motor would zing and not put down power. There are two different main shafts for the different years of a cr125, I thought I had the wrong one but this wasn’t the case. If you search this site for “5th” and the old ekarting news forum you can find the part numbers. Take a closer look at your gears again, I thought mine looked pretty good but they were indeed rounded on the dogs. It was expensive to fix but no issues with the new pieces installed. Personally, I think there is too much gear play on the secondary shaft and thought about adding a shim to help hold it in gear but decided against that. Check that there is a lot of space for the shift rod next to your seat too, I saw marks on my seat and it was flexing in fast lefts and pushing on the shift rod while on power (bad).
Christian you are right on the money with the rounded dogs. I called Mike over at shark shifter and he explained it all to me and you pretty much echoed what he told me. My shifter rod has no play in it and is super tie and doesn’t flex in either direction. With the case separated I looked at the spacing in the gears and it all seems pretty good nothing to excessive. I did take a closer look at 5th and the other cluster and it looks like I need to replace some gears. When I was talking to Mike I had him send me the parts needed to correct this as he had them in stock and could ship out that same day. I should have the parts Thursday and will compare them once its all apart.
I think to help reduce gear wear I am going to send out all gear box internals to have the REM treated by Taylor Race Engineering (isotropic finishing).