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MK3 gearbox locking

Submitted by gary_gibson on Tue, 12/06/2012
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Commando

I am restoring a MK3 and have come across a problem with the gearbox.

I have setup the quadrant and camplate and have checked I have all gears inner cover off and on.

So then I have fitter all the gears checked again I can select all gears, this has been tested cover off and cover on. Though not being a gearbox expert, it all looks like it is doing as it should.

The problem I have it is jamming in third gear (seems to happen after I have tested it a few times) so works fine then just jams. I then removed the gears until I find the jam it seems to be the selector fork on the main shaft. Tried rebuilding the box three times and keep getting the same result, my initial thought the selector is worn (which is what I had on my T140)

So before I spend the money on a new fork and find that was not the problem, is this a common problem or does anybody have any ideas?

Submitted by gary_gibson on Wed, 13/06/2012

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Took the gearbox out of the bike so I could see what was going on.

Four hours of playing around I think I have it, because the gearbox is not turning some of the gears are not lining up as they should. But when they are lined up as they should the box works, but any movement puts the gears out of line so they do not engage.

Not knowing a great deal about gearboxes, I guess when everything is spinning this will not be an issue.

Could have saved myself a lot of time and effort if I had stopped and thought about it.

Submitted by john_holmes on Wed, 13/06/2012

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As long as you can get all 4 gears whilst moving the rear wheel or the gearbox sprocket it will work on the road, without this movement you never get all 4 gears.

Submitted by ashley_cutler on Thu, 14/06/2012

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Had similar situation with my MkIII when I rebuilt the gearbox, with a nice new set of bearings and bushes in there. I was not strong enough to turn the sprocket and hand operate the selector whilst it was on the bench - fitted to bike and it tested correct when rolled back and forth so went with it as was and it is fine.

Submitted by gary_gibson on Fri, 15/06/2012

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Thanks guys feel more cofident about re-fitting the gearbox.

One other thing I fitted the outer cover, then the primary with gear change but seemed to have no gears, tried it more than three times but got the same result.

I have re-built the gearbox and will put it back in the frame this weekend and test it again is there something I need to be aware of that will stop me having gears when I put it all back together?

Could this again be that the bike is stationary?

Submitted by ashley_cutler on Sat, 16/06/2012

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Hi Gary,

As you are nearly there it would make sense to fit the rear chain and roll the bike back and forth whilst selecting the gears. The trick is to ensure when replacing the gearbox outer cover the clutch operating lever is aligned correctly and not jamming the clutch in the released position.

Hope this helps, if not where are you? someone local may be able to help.

Submitted by gary_gibson on Wed, 20/06/2012

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I have stripped the gearbox completely again including the bearings.

The reason I did this was I had all gears and was happy the box was working fine. Fitted the inner cover and the gearbox all but locked up.

To me it seems to be the kickstart shaft bush locking against 1st gear layshaft. So I stripped the box again removed bearing, measured the seat for the layshaft bearing and the bearing itself to determine if it should be proud of the case or flush. Going by these measurements the layshaft bearing should be proud of the case.

The problem I have the layshaft bearing housing has been butchered at some stage. The box does go together OK and everything seems to work.

Starting to feel as if I am going around in circles, I am in Somerset.

Submitted by david_cooper on Sat, 23/06/2012

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Are you sure you have all the gears the correct way round on the shafts? It's a while since I took a box apart, but I seem to remember that at least one gear can easily go in both ways round without it being an obvious error - but if you do so then it all jams solid. But then I don't think the cover would go on...so you'd have seen that - probably.

Do you have any bent selector forks? And are you using the manual to make sure the index plate is correct? It isn't something you can easily guess.

Submitted by gary_gibson on Wed, 27/06/2012

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Right with a little help from a friend with a dismantled gearbox and a bit of measuring we were able to work out what was going on.

First the layshaft bearing does sit proud of the casing also found the new bush 3rd gear (040047) layshaft was not going on far enough so 4th gear was stopping the bearing inner race from sitting fully home on the layshaft. We found an old bearing measured it up to check it was OK fitted this to the layshaft, the bearing inner race now sits all the way home.

I now have gears and when the inner case is fitted it no longer locks the gearbox.

The only thing now is when I fit the outer case, then being a MK3 fit the primary case I do not seem to be able to select any gears. The gear lever does have some pressure and returns to the central position, so everything is connecting OK.

So I am thinking it must be something to do with the ratchet system in the outer cover?

 


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