Gearbox.

[Gavin]

Anyone had a gearbox symptom similar to these, and if so, how did you fix it?
When I first started the bike first gear selection seemed ok, then some grating came in, then it became jammed I believe between 2 gears, Now the back wheel will only move 3/4 of a turn. The clutch started to smoke up a bit, with i suspect the rear wheel jammed at that stage. All the while the chain would grab, or flex violently. have not ridden the bike more that a few laps round the back yard, so hopefully haven't done too much damage.

[1660bob]

er-OUCH! the problem you describe sounds like some sort of catastrophic  failiure inside the box-DO NOT RIDE IT- it will likely seize or jam solid and could throw you off with potentially fatal results.Gearboxes have to be SO Right on bikes for this very reason.Strip it out if you are confident of your abilities, or get it to an expert who can repair it properly.a little knowledge can be a very dangerous thing especially where bike gearboxes are concerned, Regards, Bob.

[muskrat]

BUGGA, sounds like a bad one. Could be a multitude of things. Bob is right, DO NOT RIDE it. Could destroy other parts as well. BSA gear box problems are few and far between, they are very tough. It could be rebuilt in the frame but easier on the bench. S/A or semi unit ? Good luck, keep us posted.
Cheers

[Gavin]

Thanks. Looks like I'll try the basics again before I dismantle it. yep, I've got as far as knowing it's not a pretty problem.

[Brian]

Gavin there is one thing I would check before you dismantle it. Pull the detent plunger out of the bottom of the box and check the spring in it isnt broken or the plunger isnt stuck. The plunger is what locates the gear selection cam plate in the correct position. If its ok then you will have to dismantle the gearbox and see whats going on.

[Gavin]

Thanks Brian, I'll try that soon. Any clues as to how I might "Unlock" the rear wheel from it's 3/4 only revolution?

[Brian]

The only thing I can suggest is to put the bike on its centre stand and while gently trying to turn the back wheel pull the gearlever up, or push it down, but only a small amount, about half of what you would normally do to change gears. This is working on the theory that it has stuck between two gears. If there is a major internal problem then nothing will work and you will have to pull it apart.

[Gavin]

Ok bloke, I'll let ya know what happens. Thanks.

[The General]

I'm rebuilding my box right now.
Fairly simple construction.
If a selector was broke it might engage 2 gears at a time, but I don't think you'd get 3/4 revolution of the rear wheel.
If you got broken bits wedged in the gear teeth you might get partial rotation.
If that's the case, you'll have to drop the drive chain to push the bike.

[Gavin]

Part of the way toward an answer..... enjoy the pix.