1980 GS850G trans or shaft hum

[Moarpower]

Hi guys.
Replaced the output shaft seal and the output gear assembly for a good used one. Used a good bearing.

After reassembly all seems fine. But when the bike is warmed up and been riding for 10 minutes I start to get a him or moan. It's hard to tell if it's the shaft, rear hub or gear box. No vibration. Just a moan. Maybe the newer oil I used isn't quite suitable? I used a penrite premium mineral 80w-90 gear oil

[salty_monk]

That oil is fine. I used something similar in my G's.

A noise like that from something transmission related that's got enough oil in it.... I would say something is too tight & getting tighter when it heats up? Is there a clearance you need to set with shims at the output gear assembly?

[Moarpower]

That oil is fine. I used something similar in my G's.

A noise like that from something transmission related that's got enough oil in it.... I would say something is too tight & getting tighter when it heats up? Is there a clearance you need to set with shims at the output gear assembly?

Thanks.
It's possible that it is clearance relatively. On the output gear assembly there are 2 shims which I reinstalled.

Here is a YouTube link to ta video I recorded today to give you a good idea. The hum seems to match the revs but it's inconsistent.

https://youtube.com/shorts/MLjo3x12pIU?si=w697fHL1CZr0WjJ9

[Cipher]

Thanks.
It's possible that it is clearance relatively. On the output gear assembly there are 2 shims which I reinstalled.

Here is a YouTube link to ta video I recorded today to give you a good idea. The hum seems to match the revs but it's inconsistent.
https://youtube.com/shorts/MLjo3x12p...97fHL1CZr0WjJ9

https://gsarchive.bwringer.com/mtsac/~cliff/storage/gs/GS850_Manual_HiRes.pdf

c. Pg 119

[Grimly]

You used the old shims with the replacement gears?
Not a good idea, you don't know what the engagement pattern is now.
You can find out, of course.

[salty_monk]

Yeah that's what I remember. I think you need to use the shims from the replacement but even that might not work (if you have them you might not have a lot to lose by trying them.
The Manual shows a bunch of different clearances & Backlash figures as well as shim part numbers. Link above is a good one. I'd take a look at where you fitted the new parts.

Another things to think about is that "good used" part. I once had a Lotus Seven replica that had that same humming problem with the rear diff. What I didn't realise is that before I bought it the diff had been run a few hundred miles without oil in it (just remnants from where it had been emptied & not refilled).

It didn't matter what was done to it, that diff always whined after that. The only way I could get rid of the noise eventually was to swap it for another one. I couldn't get to the bottom of it.. one of the engineering companies I spoke to said it had "burnt gears" which doesn't make a lot of sense to me unless perhaps they were distorted /twisted from how hot they got. I didn't let them investigate further - the labour charge verses fitting another one didn't make sense, so I fitted another one.

[Moarpower]

You used the old shims with the replacement gears?
Not a good idea, you don't know what the engagement pattern is now.
You can find out, of course.

I only bought the used part for the housing itself. It didn't come with shims so I had to use my existing ones. I also used my existing bearing and gear and just mounted them in the new housing. What do you suggest ?

I also noticed that the spacer on the replacement part was 1 long spacer and 2 shims. I just used my existing spacer.

Could it be caused by just meshing the gears differently on reassembly?

[Moarpower]

I will pull the secondary drive and driven gears and check them out. I don't have the correct service tool for checking backlash tho. Any other tools I can use instead?

[Moarpower]

Wondering if incorrectly torqued nut on the output/driven gear shaft could cause it ? I'll double check it all. Just made certain that I reused all the original parts except the housing which was replaced due to a crack

[Grimly]

The workshop manual (and the Haynes) include illustrations of the meshing pattern.
This means taking it apart, cleaning it out and using engineer blue to see what / where the contact patch is.
The shiimming is meant to allow more or less engagement, and ideally a centred contact patch would run quietly.
Backlash is another story - hopefully the two gears mesh just enough with backlash in tolerance. If there's too much, the shimming can be reduced to attain the right amount.
Nothing for it, but to have at it.
If at the end of all that it still moans, lob some Molyslip in it.

[Moarpower]

Thanks mate.
I'll pull it all down today and reassemble with the gears marked and check the contact.
I would have thought that using all the same parts minus the aluminum driven gear housing would have ensured a very similar contact area but I guess not, unless the noise is being caused by something else entirely

[Cipher]

Thanks mate.
I'll pull it all down today and reassemble with the gears marked and check the contact.
I would have thought that using all the same parts minus the aluminum driven gear housing would have ensured a very similar contact area but I guess not, unless the noise is being caused by something else entirely

Have you found any sparkles yet? I can't see but is the shaft one that can be out of phase if not reassembled exactly as it was?

[Moarpower]

Have you found any sparkles yet? I can't see but is the shaft one that can be out of phase if not reassembled exactly as it was?

I've only ridden it for an hour 3 hours since I put it back together so I haven't drained the gear oil yet. I'll do that today and check for metal in the oil. It's possible that it could be out of phase as I'm sure the gears turned multiple times when I swapped the housing.

[Moarpower]

Removed the secondary gears. The driven gear has a huge polished surface so it looks like too much contact I'll post some images.