Mk 3 jumping out of 1st gear

Hello, I need some advice please.
77 Mk3, done about 10k miles and it started fumping out of 1st gear. Only now and then about 2k miles ago but getting worse till it does it 1/2 the time now.
From what I can see the 1st gear on the lay shaft can move away from the dog gear so it doesn't engage properley.
And this gear is located by the bush in the kick start shaft? I can pull the kickstart shaft out about 1mm, so I assume the bush is worn? allowing the 1st gear on the lay shaft to move away from the dog gear and not engage enough.
So a new bush and possibly a shim if needed? to stop the 1st gear moving away from the dog gear?

From what I can see the main shaft has no effect on 1st gear meshing?
The main shaft does have about 4mm end float with the inner gearbox cover removed. (and the clutch still intact)

Does only the nut on the mainshaft outside the inner cover stop end float of the mainshaft?

Recomendations and advice appreciated,,,,

Graeme

Graeme,
If your layshaft has that kind of endfloat, it has either already been converted to a roller bearing on the LH side of the shaft, or the original ball race is very loose in the case? When you convert to roller it is necessary to shim the Kick start dummy shaft, between the inner case and the KS shaft. U can use isolastic shims, If there are no shims there now, and you are no sure of the LH bearing configuration, I would gut the box and check the layshaft bearing? Wobbly 1st gear bush can add to this. Also check for play in the cases locating the kick start shaft, and the bush inside the KS that supports the right side of the layshaft.
plenty of areas for play to add up.
Hope this helps.

Cheers Richard

Yes it does thanks Richard,
Yes a roller bearing, slightly wobbly first gear bush, no shim, and the kickstart shaft is good.
So I will relpace the 1st gear bush and shim the gap between the 1st gear and the kickstart shaft.

does onything else stop endfloat on the mainshaft other than the nut outside the inner gearbox case?

graeme

You can also chck these item 30 and 29. http://www.oldbritts.com/1975_g7.html This springs free lenth should be 1.500" or 38.1mm and the plunger should be more parabolic or conical than spherical. If any less than stated, replace. This will indicate weakness as well ( should be 21 lbs/in)

You will lose your fluid when you check this but that is the worst of it. Worth a try, quick and cheap to fix. What do you have to loose, $2.44?
P.S. It worked for me,

I vote a worn thin lay shaft 1st gear bush, the thinnest wimpy-est of the bunch and maybe its mate. Pawl spring always comes to mind too.

Yes, the main shaft is located by the ball bearing on the right end . It is restrained by the nut , when tight.
The bearing is restrained in the case by the clutch release tower/nut/collar.

You might also take a look at the right-hand mainshaft ball bearing.

I once found that all the balls had been destroyed but the cages and races had held the mainshaft captive--like a gimbal or swash plate, the mainshaft end was fully bolted up but could be swiveled around the original axis with about an inch of free play all around--and just about undetectable at rest.

Replacing the layshaft first gear bush and shimming the kickstart stub to .005" end float should help eliminate the problem--good luck


Tim Kraakevik
kraakevik@voyager.net
'72 Combat, '74 RH10 850

Thank you all,
A new 1st gear layshaft bush and a new bush in the kickstart shaft and a 25 thou shim and all seem good.
Also checked the other bearings and bushes and replaced the selector spring and the small "O" ring on the selector shaft as it didn't have one. (what does the O ring do?)
Then the clutch post that holds the clutch actuating arm broke when I tightened the small nut and bolt. It seems to have been cracked as I can see different colours (colors) where the piece broke off.
I will buy a new one Monday. Do these break or is this one a dud?

Thanks for your help again,
Graeme

Graeme,
I don't believe the clutch arm sleeve is a none week spot. Maybe the PO was tempted to try and align it with a screw driver blade when it was too tight? Anyway have you had a look at the mainshaft ball bearing. Once you have undone the clutch actuator, and assuming you have the clutch pulled down to the a bare mainsaft on the primary side. There is a ring style circlip on the mainshaft, this is between where the clutch hub and the end of the sleeve gear reside. Take this off and you can gently tap out the mainshaft enough to push the bearing out of the casing for replacement. This bearing should limit mainshaft enfloat to almost nothing.
Graeme, I would also check for play in the cases the kick start shaft runs in, as all these area,s can add up to major play with your layshaft like a cock in a sock?

Cheers Rich

Hello Rich,
Could have been it cracked when I droped it of the bench? There were 2 darker marks in the steel that looked like it has been cracked for a while .
The layshaft had slide in and out movement but very little sideways movement. I did check the other bearings and all were ok, just needed to replace the 1st gear bush, the inner kickstart bush and then shim the freeplay.
I checked the clutch bearings and the primary chain alignment and replaced a weeping selector seal in the inner primary and adjusted one exhaust rocker. So a good weekend.
Thanks for the advice to check the other bearings.
What sort of life should I expect from these bearings? (gearbox and clutch)
Graeme