kickstart lever

There's been threads talking about the layshaft bearing.
After taking a read to them I saw that whenever the kickstart lever pulls back under acceleration when in 1st gear means the bearing is about to collapse.
I've replaced mine 4 times and the problem is still there... I've checked the bush into the pawl and it looks alright to me... I've locktited the bearing.
What else could make the lever pull back?

I'm really scared to ride the bike crossing my fingers and pulling the clutch lever a same time :?

What bearing are you putting back in, the 2 workable options are the roller one or the uprated ball recommeneded by Mick Hemmings. The roller one should have a C3 clearance ie slightly extra clearance to allow for the compression of the alloy casing on the steel bearing outer due to theinterference fit.

The kickstart lever moving downwards under accelleration is indeed a classic symptom of drive side layshaft bearing failure, although I think it might also apply to the timing side bush if the layshaft was picking up inside the bush I suppose? Did you check if the layshaft and bush clearance is adequate? As the layshaft bush may require reaming once fitted.

Your concerns appear to be with the timing side layshaft bush, and it isn't clear from what you've said so far about which bearing or bearings you have changed each time, so could we have some more detailed info please?

You mentioned "1st gear"?

Does the problem only occur in 1st gear?
As I would expect it to happen in all gears under hard accelleration, and not just 1st gear if it was the drive side bearing breaking up? If the problem does only affect 1st gear then it might suggest a lack of clearance between layshaft 1st gear and the kickstart shaft when 1st gear is selected?

could we have a weak or incorrectly fitted kickstart spring here this is a new one on me

I had the same problem for many years even with a new roller bearing and kickstart spring. What happens is when 1st is engaged, the four dogs on 2nd gear extend through layshaft 1st gear and contact the kickstart shaft face with enough force to rotate the K/S. If you look at http://www.oldbritts.com/ob_start.html first picture on 'how gearbox works' in 'Technical-Gearbox' you can see the marks on the dogs.
Why it is only a problem on a few bikes I do not know, extreme tolerances I guess and bad luck? Mine was originally solved with shims between the 1st gear and K/S bush. However you must be careful and only add just enough to prevent loading of the K/S shaft or the gears will seize. You need to examine the parts and experiment with shims.
I later had the layshaft modified for needle race with the expensive layshaft ball bearing but that would not have resolved the 'moving' K/S. I still needed shims to prevent the problem.
Alternatively, I thought of grinding a few thou (0.25mm?) off the 2nd gear dogs since there is a lot of engagement. At worst you ruin an expensive gear if it fails and I was not sure about heat treatment but as the gear only engages at relatively low speeds normally you might not need to reharden the gear if only a little material is removed.
Apologies for the long post but it had me confused for several years and as you say is really annoying.

Thank you all, mates, for the great feedback.

The lever moves when I accelerate in 1st gear (doesn't seem to move when I engage any others) I've changed the drive side bearing and always used the FAG roller bearing provided by Andover except one time I tried a Torrington with same results.

I always thought (as I've read in other threads) this kind of bearing needs shims like the ones on the rear isolastic to adjust the excess of clearance between the pawl and the case, but never tried to do so.
This case seems to be the opposite as L.A.B. gently points, but, in fact, I removed a suspicious shim there was between the layshaft first and second gears, as I reassambled the gearbox following Hemmings DVD, as he doesn't put any shims there, so there should be a little more clearence between 1st and the pawl.

I've never changed the timing side bush... How should I ream it when I replace it?

If it helps, after reassembling the gearbox, not only the kickstart lever kept on pulling back as it did before, but 1st gear began to jump out (so it works even worse than before with the "old" bearing)

Any help is really appreciated.

The timing side (K/S) bush does not need reaming but it needs to be tapped and pulled out, it could be very tight. Or you could use a hacksaw blade and saw through it carefully.
As I said it's likely the 2nd laygear dogs are driving the K/S. A new bush will not help. I've been there and done it all!!!
The fact that you removed the 'suspicious' shim indicates a previous owner had tried to solve the problem. A shim between 1st and 2nd would reduce the engagement of 2nd gear dogs I believe and reduce contact to the K/S shaft. Yes there should be no shims there but the manuals and videos are only OK in a perfect world. RGM helped me out with the problem when I had identified what was happening.

Thanks Keith!
In that case, that would mean the dogs are pulling the lever because they get really deep into the holes of the 1st gear, so why it jumps out of gear then? I suppose it should stay into gear even more than before removing the shim?

Yes the jumping out of gear is much harder to resolve and you are correct, removing that shim should improve engagement.
It could be - Too much axial gear movement allowing the dogs to drop out or a badly worn selector plunger and or selector track. However I don't think the gear can move that much between the 2nd gear and the K/S shaft face.
Going back to the original question about the K/S rotating. If your bearings are in good condition there is no danger of the gearbox locking up. It is just a really annoying problem when starting off. The harder the start the worse it will be. Gentle starts usually improve it. That's no solution but is how I lived with it for some time.

Thanks everyone!