426 Shifting problem

I have a 426 converted to supermoto. Problem is that at high speeds, shifting from 4th to 5th, sometimes the bike will run in 5th for a short time and then fall back into 4th on its own. Sometimes she will bounce between 4th and 5th as well, until I shift again. Basically she'll shift on her own from 5th to 4th when upshifting from 4th or she'll jump between the two gears on her own when upshifting from 4th, both under power.

This is not a constant problem, but it is most evident on the front and back straights when I'm getting into the higher gears at higher speeds (in higher revs).

Am I on the brink of transmission failure? Is there something to adjust? Could it be the oil (I heard that some types can make it slip)? Anything I can try short of splitting the cases? I am slightly mechanically inclined, but pulling the motor solo is probably beyond my skill level.

Thanks!

~Jeff

It sounds like you might have a bent shift fork or the gears in the trany are wearing out. If you have a bent shift fork the gears might not be engaging all the way and slipping out of gear. As far as I know there is no way to do any thing about it short of diving in to the motor.

I think that the oil causing slippage that you have heard about is in the clutch. Certain additives are not good for wet clutches and can cause them to slip. Not the gear slipping that you are talking about.

Jeff, I had exactly the same problem, as have quite a few other TT riders. In my case the degradation happened a lot quicker. At the start of the ride I had your symptoms - by the end I couldn't get 5th at all.

Turned out to be a worn shift fork, which in turn caused wear on the gears. You'll need to pull it down and replace worn gears and the shift fork. Don't know how to upload pics, but I've still got the shift fork and you can clearly see the wear. Someone else has posted pics of this same problem though on TT.

Jeff, I had exactly the same problem, as have quite a few other TT riders. In my case the degradation happened a lot quicker. At the start of the ride I had your symptoms - by the end I couldn't get 5th at all.

Turned out to be a worn shift fork, which in turn caused wear on the gears. You'll need to pull it down and replace worn gears and the shift fork. Don't know how to upload pics, but I've still got the shift fork and you can clearly see the wear. Someone else has posted pics of this same problem though on TT.

Thanks guys! I will do a search for warn shift fork and see what I come up with.

Curious if this could be a symtom of my problem. After 2-3 hrs of riding, or long climbs in 1st and 2nd, I have difficulty shifting between 1st and 2nd, usually goes into nuetral. Frustrating when on long climbs losing that momentum.

Any ideas?

your oil viscocity can always alter the way your tanny gears ove with one anoher......

always keep your oil changed more than regularly...

if you havent changed it in a while of if you put same oil back in cuz you thought it looked new still.....then change it....

just 2 qts of oil.....

i like to run the thicker oil 20-50 yamalube.....

most likely you havea worn shift fork tho...

happy wrenching....

Dog clutches on the back of 3rd and 5th gears are most likely worn. You might have a bent shift fork too. Time to split the motor. Whilst you've got it apart, there should be a toolmaker in your area that can "back cut" the dog clutches on the gears so that as they try to slip, they're forced to engage more. I went through it with a WR400 with a 426 kit. I split it twice before I found the backcut fix. Now with an 09 WR450. First split of the new motor, the gears will be backcut.

Mark

Good idea with the back cutting. I used to do that with the old B-W T-10 transmissions in my car racing days.

Mine went quick too. It was a long 200 miles back to the truck in 4th gear only.

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