Barnfind won’t shift

Hi, forum!

Continuing work on a ‘73 750 Commando that was at rest for the past couple decades. Posted previously about starting/running problems traced down to bad condensers.

Situation: unable to shift out of fourth gear.

Background: as described, bike was languishing in the back of a repair shop for decades with minimal available history beyond “it was very fast, was being set up as a race bike.” Notable that there was no drive chain installed; there WAS a chain coiled up under the carbs, but upon fitting it to the sprockets, it’s three links too long!

Assessment: got the engine freed up and running; clutch is still stuck (will kickstart with the lever pulled in), and the counter shaft sprocket turns with the engine.

The shift lever moves down easily, limited travel up with no discernible engagement of gear selection. Dunstall rear sets are fitted with a parallelogram shifter, so the gear indicator has been removed, but the indicator fixing bolt doesn’t rotate in either direction, either with the lever movement or gently turned with a wrench while rotating the rear wheel (chain’s back on/spark plugs removed

Drained the gearbox oil, no metal shards noted….my GOD does that stuff reek.

Removed the outer cover and can see no obviously broken pieces, although I’m not sure I understand exactly how the pawl/hairpin arrangement engages to advance the ratchet plate assembly.

The quadrant is at the top of its arc, which suggests fourth gear, and does not move with judicious leverage applied while rotating the rear wheel.

Recommendation: Ask the forum for wisdom/advice/guesses/wagers before plunging ahead and stripping the gearbox further. What’s likely to have broken to allow the shafts to turn, gears to mesh but not transfer shifter motion beyond the quadrant? Is it possible that the cam plunger is frozen and locking the cam plate? I don’t KNOW that the missing chain means the ‘box had failed, leading to the three decade parking ticket; could it have packed up over time?

I’ll have to take another hard look at the spring/pawl as well, need to understand how this works.

Clutch tool on the way to tear down/clean/free up the clutch.

Any and all input cheerfully welcomed!

Marc

ps: when installing a chain when there’s no old chain to pull it through, a .46 wound guitar string works great…can be pre-bent to thread it through!

MarcD said:

The shift lever moves down easily, limited travel up with no discernible engagement of gear selection.

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The pawl spring possibly needs resetting see page 2 of the Oldbritts gearbox article: "What happens when you press on the shift lever":.

>Download

Have you tried turning the camplate bolt while also turning the rear wheel as that should select all gears? The camplate bolt and therefore the camplate should turn with comparatively little effort. If, however, the camplate does not turn reasonably easily (from the top gear position?) then that could indicate something wrong inside the gearbox.

I would not be comfortable with this box without a complete teardown and at minimum, a new layshaft bearing installed.
The layshaft bearing is a known weakness on these. When they go, they lock everything up. May be your problem.
These boxes are not that complicated, and allows you to get to know it.

These gear boxes are not as complicated as you may think. Follow LAB advice but there are plenty of exploded views to help you along. If and when you remove the outer cover and find any rust on any steel part you will have to completely disassemble gear box. After years of sitting a BSA of mine had rust inside that would have been very expensive if I had tried to run it down the road.

Good pictures are helpful for diagnosing advice

Wow, thanks for quick the quick responses!

L.A.B…good advice on OldBitt tech article; those articles are a treasure trove. I seen the explanation, I’ve held the parts in my hand and played with them and still awaiting that “aha” moment . The fact that the quadrant appears to be frozen suggests deeper problems than the shifter.

As I mentioned, I did try shifting by turning the gear change indicator bolt while turning the rear wheel, which was unsuccessful. Where will I find the cam plate bolt you suggested? Not seeing on the exploded Haynes illustrations.

MichealB…Thanks for the layshaft bearing note; I’ve read a few horror stories about it. Looks like this box has never been apart, with 7k on the odometer (who knows how accurate), but this would be a good time to replace it. The shafts currently turn, the gears mesh and the Kickstarter isn’t drooping, there’s no debris in the oil… it just won’t shift out of gear

Mean Gene…I have had the outer cover off, and no rust so far. I’m thinking a complete tear down is inevitable, even if I can find a quick fix for the stuck shift.

Keep ‘em coming!

Marc

BERT said:

Good pictures are helpful for diagnosing advice

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You’re right. Time to pony up for VIP!

MarcD said:

Where will I find the cam plate bolt you suggested? Not seeing on the exploded Haynes illustrations.

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Youll find it easier if the box is , on the bench . But youll be at it longer ,
but it may be quicker in the long run .

The General - If you know its done right , you shouldnt have to do it AGAIN . ( plural ) .

Oh man, that did it! Hadn’t put it together from the two diagrams that the two #26bolts hold the quadrant and cam plate in their respective bushings!

Just a little nudge with a stubby wrench and was able to shift (with turning the rear tire and get through all the gears! The gear selector still isn’t working right, so I’ll take the outer cover off again and stare at it until operation becomes obvious.

And then I should tear down the entire box and properly rebuild it. And understand it.

To paraphrase Ol Brit “it becomes intuitive once you’ve repeated the task several times”, or something like that.

Can’t thank you all too much!

Marc

mean gene said:

These gear boxes are not as complicated as you may think.

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I wouldn't say they are a walk in the park either, especially if you mess with the cam plate. That said, a rebuild is probably warranted, especially if you have original bearings.

There's a lot of posts on rebuilding gearboxes. My fun is below (starts at post #149 and goes through #220). Good luck

Other useful gearbox info. I like the animation pdf.