Vibrations on deceleration or clutch operation

[ned]

This happens only when the bike is thoroughly HOT, ie had a 30+min
on the open road. Never on short runs.

Under power all is well, but on deceleration or clutch operation the bike turns into a buzz saw until the throttle is reapplied. I can feel vibrations all over the bike, so I suspect that it comes through the engine mounts.

clutch problem, internal chain, oil too thin (5w-40)?? Any thoughts?

Ned

[Placerville]

There are several things that could be causing this but, with only what you've stated, it's difficult to diagnose. My first response is regarding the oil you're using. Is there some reason you're using a 5W-40 oil?

Your '76 requires 10W-40 motorcycle oil (or a conventional grade oil with no friction modifiers such as Shell Rotella T 15W-40. Go here to read more on this high quality oil that's a favorite of many vintage GL owners. If Shell Rotella T is not available in your part of the world, any good quality 10W-40 motorcycle oil will work. I would first change your oil to ensure that your clutch is not affected by the oil your currently using. Your statement about "heat", "clutch operation" and "deceleration" make me suspect your issue may be oil/clutch related. If this has no affect, start looking at drive-line issues.

[ned]

There are several things that could be causing this but, with only what you've stated, it's difficult to diagnose. My first response is regarding the oil you're using. Is there some reason you're using a 5W-40 oil?

Your '76 requires 10W-40 motorcycle oil (or a conventional grade oil with no friction modifiers such as Shell Rotella T 15W-40. ...

I would first change your oil to ensure that your clutch is not affected by the oil your currently using. Your statement about "heat", "clutch operation" and "deceleration" make me suspect your issue may be oil/clutch related. If this has no affect, start looking at drive-line issues.

Used 5w-40 diesel oil (JASO MA rated) to flush the spagg clutch ... BTW it worked great. As the oil is only 500 miles old I decided to take the bike out for a good long run before I change it. I do think that temperature related oil viscosity is the problem because I can ride it for 30min or so without any hint of this.

Unfortunately Shell in it wisdom have changed the name of their Rotella product to something like Rimula ... I need to workout which has the JASO rating for wet clutch application.

Good plan. I will change the oil before I take the girl for another run along the same road just to see if the problem is reproduced. I tend to agree that it is a simplest test.

[Placerville]

Agree.

Do Aussies have Marvel Mystery Oil available? If so, it's a good product to use as a crankcase & sprag clutch cleaner. Replace 1 qt. of your oil with MMO prior to an oil change. Take a 100 mile easy ride with nothing over 4K rpm then drain and replace with fresh oil. If MMO isn't available, SeaFoam would work or, as a last resort, ATF.

I was suggesting this as a way to clean out your old oil and as an alternative to using your current method. Good luck!

[ned]

Agree.
Do Aussies have Marvel Mystery Oil available? ....

Yes, I think so, but in a few speciality auto parts store.

[ned]

Just to sum up

Changed the 5w-40 oil used (very successfully as a flush) and replaced it with 15w-50 (yes, I know) fully synthetics, SHEAR FREE, JASO MA rated for wet clutches and took her for an identical ride along the same road.

All problems disappeared and she is running much better, smoother and without signs of vibrations.

Now, I looked back on 5w-40 oil. In principle 40 is 40 when the bike is hot, yes? The difference in viscosity between 5w-40 and 15w-40 at 100C should be the same, but obviously this is not so. The problem was, I believe, that viscosity modifiers (Thickener) were sheared (destroyed) by the clutch and gearbox operation and as the result of this the oil thins out far too much, even after 500 miles. I think that that is the reason 15w-40 is recommended and works well.

Ned

[Placerville]

Congrats. Always nice to see something easily fixed.