third gear

boz · Apr 21, 2011

Even though I have owned my bike for 3 years I have not ridden it much if at all. This week I was able to get it running after a head rebuild, a rebore and new pistons and rings and replacing the carbs with a single Mikuni. Bike sarts and idles great. I have begun taking short trips around the shop to heat cycle everthing and do some gentile breaking in. I notice that shifting 1st to second no problem but 2nd to 3rd if I don't completely remove my foot from the shift lever before I try and shift into 3rd no go. If I forget I have to let out the clutch run it in second with the foot off the lever then try to up shift again and it will go. Nature of the beast for these transmissions or should I be looking at something? Oh by the way the bike is a 1970 750 "S".

79x100 · Apr 21, 2011

The problem probably lies with the ratchet assembly. perhaps a worn or tired pawl spring. It's worth taking the outer cover off the box and having a look at the springs and their alignment. A search of the forum under 'Pawl Spring' will find you lots of info.

DogT · Apr 21, 2011

The nature of these gearboxes should be to shift right every time. I never had an issue other than the mainshaft outer bushing got eaten up by the circlip and I would miss neutral once in a while.

Dave
69S

Hobot · Apr 21, 2011

Suspect, worn dog ears, pawl spring rusted out of parallel setting, worn bushes giving sloppy cog engagement. Nothing for it but dig in and see and while in there renew the rest of it.

DogT · Apr 21, 2011

Would be a good time to inspect the infamous layshaft bearing while you're in there. Look it up on the forum, there is plenty of info on it.

Dave
69S

BrianK · Apr 22, 2011

The pawl springs being somewhat notorious on these bikes, and so easy to check, in situ, that that would be my first thing to check. You probably want to keep a spare one on hand anyway, so have one ready to install and if you don't need it, perhaps tuck it into your toolkit or onboard spares kit - best way to ensure you'll never need it!

Which is not to say you shouldn't check and replace the layshaft bearing if you're not 100% sure you don't have one of Portugal's finest ball bearings in there.

Best of luck.

chris plant · Apr 22, 2011

hi boz,before you go stripping the box try removing your foot from the gear stick after every change,it lets the pedal centralise ready for the next change,if the pedal does,nt centralise then suspect the return spring

boz · Apr 23, 2011

I can make the upshift if I completly remove my foot from the shift lever. The strange part is it only happens when making the second to third gear upshift. First to second and third to 4th all good as are all the downshifts.

batrider · Apr 23, 2011

Try fooling around with the lever position and see if there is a spot where your foot is not touching the lever between shifts and also is comfortable when riding along in 4th gear. My foot normally is below the lever because I want to be ready to downshift. Maybe that's weird. I moved mine last year because I went to a couple group rides and people would bunch up when braking -- someone went down because of it. So I felt downshifting quickly was more important. But now I just avoid group rides.

Hobot · Apr 23, 2011

I had 2nd>3rd shift issues once that was caused by the forked shifter dogs faces wearing smearing, so replaced. I'd fixed a number of things prior so nothing for it by operate.

possm · Apr 23, 2011

I have had the same problem with a 70 asuming the bax has not been apart and therefore is "timed proprly", the broblem lies in the outer cover, !st make sure that both springs are in good condition and the that the one that looks like a hair clip is positioned with the bent portion facing down, if you refer to an early manual it advises that where the mechanism bolts to the outer cover that it may be neccesscary to elonggate these holes for correct alignment,
I found with mine it was not nessescary to elongate but there was enough movement to gain correct alignment. I spent several weeks and anumber of full gearbox strips before i found this out.
Al

Hobot · Apr 24, 2011

I've had the pawl spring become rusty and somehow not work the pawl so shifts were hit and miss. Then took a few springs diddling to discover they may need diddling to get right out of the box. Trick is both legs parallel in pawl arc area and enough gap pawl just passes freely to test. Crook to bottom was not the way I did it first time, but now I know better : )

boz · May 25, 2011

I removed the outer cover and inspected the pawl spring and it shows no wear or rust and is straight and has the just perceptable clearance as called for. However I noticed that the rachet plate that the pawl engages has a post that engages a round barrell that moves the shift fork. That post is worn egg shaped and I suspect this to be my problem. Any thoughtds on this?

Hobot · May 25, 2011

That post is worn egg shaped and I suspect this to be my problem. Any thoughtds on this?

Ugh, good observation and likely the cause for miss shifts that are absolutely not part of good AMC gearbox character, which should be the best in any tranny ever made for cycles. Nothing for it but complete tear down to replace that part and see what happens. One thing Commandos taught me is very rarely can ya avoid full efforts a few times to get full joy in the end.

third gear

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