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