AMC gearbox 2nd gear

I would replace the pawl as well as the selector springs. Make sure you have the spring that touches the pawl the right way round.

Previously spencer_hunt wrote:

My1975 Mk3 Commando is developing a gearbox problem in that as it warms, it occassionly wont let me select 2nd gear. Shifting back to first and reving it up to about 3000rpm will ususally get around the problem. Last ride out I had to stop the engine, select 2nd gear then restart to get going. It has the roller bearing mode done by a previous owner. I replacedthe 2 springs in the selection mechanism leaving the pawl with a little freedom when I got the bike and has gone well for the last 3 years. I have the cover plate off and all the selection mechanism looks OK. I have the bike up on my bike stand at thepresent and rotating the rear wheel it easily changes from gear to gear. Any ideas before I pull it down further?

Have you checked clutch adjustment? Adjust to only a quarter turn on the push rod adjuster, check cable adjuster. When did you last change primary drive oil?

What layshaft bearings have you got? Check end play on kick start shaft.

Look up Old Brits Norton and look at their gearbox Technical advise.

May be worth stripping gearbox rather than ridding if you do not find the answer, being on a bike when the gearbox seizes is not fun.

Hi Spencer. If you look at my gearbox diagram and operate it via your mouse wheel, you will see the change of cog locations that are occurring. You can see that the 3rd gear on the main shaft is moved to mesh its dogs into the second gear dogs on the main shaft. These might be worn or burred.

Obviously if any part of the quadrant, camplate and selector mechanism is failing to push the the 3rd gear over enough than engagement will be uncertain.

There is a small chance that the gear-change quadrant is meshing one tooth out on the camplate selector cog..... This can be checked if the outer cover is removed...no need to remove inner cover.

Please see the file attachment...nortonamcgearbox-pdf

PLEASE NOTE: AFTER DOWNLOAD AND OPENING, SAVE THE FILE ONTO YOUR COMPUTER AND OPEN THIS SAVED FILE. YOU WILL BE ABLE TO "CHANGE GEARS" WITH YOUR MOUSE THEN FOR A BETTER UNDERSTANDING OF THE ACTION.

Les

After looking at Les's gearchange pdf (well done that man !!) a 1st to 2nd gear only problem looks like it could possibly be the 2nd sliding gear on the layshaft, if its stuck and cannot unmesh from the first gear, too much undercut on the dogs could give you that or the first gear dog holes have wear from the kickstart ratchet running inside of it.

Well spotted John, yes, that is another possibility, obviously made worse if there is slight clutch drag which prevents any drive slack to be created that allows the 2nd sliding gear dogs to pull out of the 1st gear dog holes.

If you do need to do a full strip I think this write up is nice and clear and note photo No 3:..it shows the quadrant alignment for 4th gear.

http://archives.jampot.dk/technical/Transmission/AMC/AMC_gearbox_rebuild.pdf

Les

Swapping the layshaft ball race for a roller is what gives the excess shaft float. You will need to fix with isos shim at KS end. Open out the bore of the shim. I had a similar problem on a Dommy from the clutch spider bearing down on the sleeve gear ,don't know if the later bikes can get something similar.