Naughty Gearbox

[andrew.chapman]

While riding my VH on Sunday, there was a sporadic hesitation, not unlike a misfire, while accelerating. However, a process of elimination showed that it only ever happened in second gear and it got worse with harder acceleration. It felt like gears skipping teeth inside the box. Strangely, it isn't particularly noisy when it does it

My guess is that it is something to do with worn dog teeth but it also occurred to me that it might be something to do with the selector mechanism not meshing the teeth properly.

Has anybody had this happen to them? Any guesses about what it might be?

[adrie.degraaff]

I think your crank slip is doing his work, in that case the spring is to soft or the slip worn.

For my self I am using a bush so it can't slip fully, just move.

[Steven.Carter]

I have had a similar problem on one of my gearboxes Adrie. do you mean clutch slip? because when I replaced my disks it did seem better but I did still seem to suffer from Andrews problem,

Steve

[david.anderson]

Andrew
If you think you have a gearbox problem then I would not ride the bike under any circumstances. If you have broken teeth in there and a piece locks between gears your transmission will lock and you will skid to a stop. If you are at speed then you had better hope that your rear chain breaks.
If you hear a clunk with each slip and it is in the gearbox it is possible that the layshaft bushes are so worn that the layshaft is moving over and the gears are not engaging properly. I recently repaired a gearbox where the layshaft bush was worn through and the shaft was just starting to rub on the casing.
Another possibility is that either your engine or gearbox sprocket is so worn that the teeth have worn too thin as to be breaking off so you get a bit of a slip of the chain before the chain engages on the next tooth.
I would also check that the engine sprocket shock absorber is not worn and slipping over the top.
With your clutch make sure your clutch pushrod length adjusting screw is set right and not binding and that you have a little slack in your cable as that will give clutch slip.
If it only happens in second gear though it does sound like gearbox.
David

[nevhunter]

You can get a lot of lost motion in the selector forks. The pins can wear and the selector fork itself can wear on the sides. ( It is aluminium bronze) You can also have end float . All that plus a bit of wear on the tips of where the teeth engage and it can slip out of engagement. If your gear box has had the grease go hard, these horrible things can happen. I always add enough oil so the grease will churn.
Since it only happens in second gear i would look at the gearbox first, until then I would have looked at the engine mainshaft cush drive. You have to use a very strong spring with a good VH and adjust it so as it can't possibly go over the cam . It has to bottom out first. Sometimes you need a spacer (as has been mentioned).. Nev

[adrie.degraaff]

By crank-slip I mean shockabsorber, could not find the word.

[andrew.chapman]

Many thanks guys.

All of the sprockets are in good order and the chains are fairly new and correctly adjusted. I am quite certain that it is nothing to do with them. The shock-absorber is a possibility but why would it only happen in second gear? It happens under even very light load and it doesn't happen in 1st gear, however hard I (try to) accelerate.

It first happened when I hooked up into second gear to overtake a slow vehicle. I had been stuck behind it for some time so there is the possibility that I was a little more aggressive with the down-change than usual. Then it was full throttle until I felt something give and the drive was intermittent until I changed up to third. Then it settled down but it recurred when I used 2nd gear again. Why do these things always happen when you are trying to overtake? I used to have a misfire (now cured) that seemed only to happen when I tried to overtake.

I will check the shock absorber, of course, but I am 78.7% certain that it is the gearbox. I was looking at some detailed pictures of the internals of the gearbox yesterday and it seems that there are a number of possibilities. But it probably is cummulative wear in multiple components that has done it.

I have a spare gearbox so I may rebuild that and do a swap.

[p tuxworth]

does point to box but i would have thought it might have made a noise , is the primmary side crank pin a tapper fit type with no key ? this would be the last thing to look at i know and im not sure of the set up

[brenton.roy]

Hi Andy, I'm a bit (ok, a lot) out of my league here, but I did have this problem with another make, and it was cured by having the dogs on second gear undercut .
Second seems to go first (no pun intended).
If there is no major wear in the selectors or other bits, as above, this may be worth a try.
Brenton

[david.anderson]

Andrew
If it is due to a little wear and the gears not being undercut as mentioned by Brenton then the tell tale sign in the gearbox will be blueing of the selector fork, that is if I am right and you are referring to a GB gearbox in a 54VH. The selector forks in a GB box are steel rather than the aluminium bronze of the BA box. When wear develops in the dogs, which is made worse by worn bushes that allow the gear to rock on the shaft, the dogs that engage into 2nd gear try to disengage. (Undercutting makes the dogs pull together into full engagement.) With the worn dogs trying to push the gears apart, the only thing stopping disengagement is the selector fork. Under load the end of the gear pushes hard against the side of the selector fork, often wearing the selector fork thinner (resulting in more of the lost motion that Nev mentioned) and because of the end thrust the heat build up will turn the selector fork blue. I have a couple of thin blue selector forks that were part of a Matchy gearbox that I was robbing parts from for my HT.
That said it is more normal in such instances where the dogs are trying to come out of engagement and are pushing on the selector fork, for the selector fork to spin the gear selector drum so that the bike will just jump out of gear into a neutral between gears rather than remain in partial engagement. My main suspect is still excess wear in layshaft bushes.
If you are going to install another gearbox I would first replace every bush in the box (and the ball races for good measure). How many GB gearboxes have a 3rd gear whine? Worn layshaft or gear bushings will only wear out good gears.
David