650 Gearbox - Long Story!

[G0FHM]

Hi Chaps...

Some of you will already have seen me banging on about our '81 650 Kat, and the gearbox problems.

For those of you that haven't, it had a fault with second gear whereby immediately on hitting 3500rpm it would repeatedly jump in and out regardless of how much gas it was being given (so that kind of rules out under heavy load), with a loud clack as it was jumping in and out. It has an indicated 69k on the clock.

I've bought another gearset (from the US, 21000 miles, in bloody good nick), and have split the cases to change the clusters. However, on splitting the casings I can find no real reason as to why it is jumping out!

However, I have noticed that the end of the layshaft (clutch end) was turning in it's bearing, giving approximately 1mm of lateral movement on the shaft (take the shaft out, and it drops off the end).

All the dogs look to be in good condition and not chipped or excessively worn (the ones that I suspected, the T shaped ones on the mainshaft actually come into play when 4th is selected). There is some wear on the shafts or the inside of the gears as the gears are fairly loose on the shaft and can be rocked back and forth.

I've selected 2nd gear, and try as I might, I cannot make it jump (ie using finger pressure to try and prise the gears apart).

There is some movement between the shift forks and the grooves in the gears (I'll look at the manual and get the feeler guages out) and also between the "nipple" on the shift fork and the groove in the selector drum, but not excessive.

Second gear had a really nasty whine, but the others didn't. Which bearing would be causing this?

At the final drive end of the mainshaft, there is the big spring and the 2 bits that look as though they interlock, bit could "let go" if needed. Is this a built in safety feature to allow the back wheel to still turn if the engine should lock up for whatever reason? Could it be that?

And lastly, when I took it to get it set up, Shaun road tested it. He said that if it was ridden hard, the clutch would slip at the drop of a hat. We haven't ridden it hard, as it's still running in... Could it be conceivable that the clutch was causing the problem? But if it did it in second at 3500rpm in second, why is it not doing it in other gears?

I've just bought a used drum, forks an all from a breaker in the US via ebay for £37 including delivery. I'll change the bleedin lot while I have got it apart!

I wouldn't mind some thoughts on the above though, just to help me understand gearboxes (I'm a gearbox virgin), and help me "come to terms" with it!

Cheers,

J

PS, I'm aware that 0.5mm here and 0.8mm there can soon add up to excessive clearance somewhere else....

[callyboy]

What oil you using mate ?

Pete

[G0FHM]

10w40 semi diesel... The bike has only done 120 miles since we bought it, so lord alone knows what was in it for the other 68734 miles! I'm aware of the oil situation with them, basically bunged in what I had in the shed to clear it out after a top end rebuild, as it was quite mucky inside.

The clutch needs replacing anyway - There is still "feelable" meat on it, but it is still pretty low. Couple that with oil that is probably too slippery for a wet clutch (I think it was API SL where as the Silkolene I bung in my Triumph is API SG spec).

I'm pretty sure it isn't the clutch though... The autopsy is continuing

J

[G0FHM]

Well I think I have found the problem....

https://www.flickr.com/photos/125477655 ... 498702390/" onclick="window.open(this.href);return false;

No wonder it was jumping out of second! That's the 2/3 fork....

J

[Kryten]

Always good to find a "Smoking Gun"

Now all you have to do is put it back together....

[G0FHM]

Well, the replacement shift forks were ordered this afternoon, and are in the USPS system already, in Utah!

Hopefully they should be here by this time next week, going by how quickly the gearset came.

Just got to try and fathom out how to get it together again! Looks like it could be fun

J

[callyboy]

Nice one. Give the old girl a treat and stick some mineral oil in her

Pete

[G0FHM]

Just an update:

Finally gotten round to getting it all back together (wasn't as big a headache as i'd expected), and took it for a blast this afternoon.

No gearbox maladies! Hardly any whine either! Went up to 5k RPM in second, and no jumpy outy!

All filled with Rock Oil Gamma 10w40 mineral, and all appears good.

The gearset and forks were from a 21k mile GS-650GL.

The only problem now, is instead of being a floppy change, it's now quite precise.... And hitting neutral is a bit of an art form! From first it kicks up into second, then you have to tap it gently to knock it into N.

Should be fun when Clair gets on it..... Every bike she gets on, the "gearbox fairies" pay her a visit, and she either gets a box full of neutrals, or no neutral at all!

Watch this space

J

PS, before I did the work, I was quite concerned at how hot it was running (smoke billowing off the engine hot). I've retarded the ignition timing a few degrees (as much as the slots in the backplate will allow) and all appears good now!

[johnr]

check and adjust the clutch again. the gearbox on the gs is precise and neutral is easy to find. if its tricky to select then its almost always because the clutch isnt adjusted correctly.