Strange Vibration

This is a question I asked a while ago and I tried all the sugestions but it still has an "intermitant" vibration felt mostly through the foot pegs, but also the seat and bars.

1977 Mk3, rebuilt with new bearings and iso's and etc.

This vibration is only in 4th gear and only above 3800 rpm.

It will start instantly without the revs or speed changing or bumps and isn't always there.

It does not do it in any other gear, regardless of the rpm.

Sometimes it doesn't appear at all, but mostly it does. Sometimes it goes away and then comes back like it is being switched on and off.

It doesn't appear on a mountain fang only when cruising. (maybe I don't notice it when I'm having fun)

The only way to stop it is to slow down to about 3000 rpm.

I can live with it but "What Is It" where is it comming from????????

I have tightened and loosened the front and rear iso's and the head steady. Doesn't seem to make any difference. (to this strange vibration)

??????

Triplex chain ?

Nothing touches the frame and vibrates.

Graeme

If it only happens in 4th then that is the only gear where the mainshaft is locked to the gearbox sprocket/4th mainshaft gear, all the rest of the gearbox internals are spinning but have no load on them. Maybe they is a misalignment in the gearbox or a worn bush.

Any chance it is coupled to the higher rotational speed of the wheels? Besides the gearbox that is the only other thing I can think of that changes in fourth gear.

Does it still happen if you pull in the clutch while it is happenig?

The gearbox was rebuilt with new bearings and bushes. It starts vibrating instantly like a switch is turned on to vibrate then can stop as instantly. and it doesn't always do it.

The wheels (as on the bike when I bought it) are 18" Akrons alloy rims laced with stainless spokes to the standard hubs. and with new bearings.

I haven't tried pulling the clutch in while it is doing the strange vibration, but I will try this today and answer your question.


The thing that makes me suspect the primary chain it that when I fitted the chain case inner and the clutch was that the primary sprocket and the clutch sprocket they were 8.5mm out of alignment.
I had to remove the spacer from behind the clutch and grind some of the inner chaincase to allow the clutch to sit closer to the gearbox to get the sprockets in line better than they were. The best I could get them was a litle under 3mm out of align, still with the clutch proud of the main sprocket.
The frame, swingarm and cradle were all checked for straightness before they were painted. (by a trusted frame engineer, Dave Kellet)
I assumed the machining on this '77 Mk3 was not as it should have been when it left the manufacturer/reciever. (as the machining on the cylinder and head were very poor)

The primary chain and sprockets are the originals as far as I'm aware. (17000 miles)

Graeme

Could it be the gearbox spacer on the inside of the cradle between gearbox and cradle part # 03-0023 Phil

So many things can cause same symptoms.
On - off like a switch in particular frequency range-loading,
but with random onset - implies to me an electrical gremlin.
Mechanical source would be expected more cyclic or
constant and reproducible.
So could be the mechanical fault is jostling the electrical.

Thing that make one side of engine combust differently
from the opposite are felt more than whole engine
running uneven.

Odd ball case could be rear chain riding up on sprockets
if chain well worn and slack enough reacts to road rhythm.

Commandos should be heard not felt.

hobot

Norton-Villier, It could be the wrong size spacer, but if it was too thick, the other side of the gearbox would need a spacer to take up the slack.
The gearbox with it's spacer #03-0023 fit nicely in the cradle.

Highdesert, been for a ride and after sitting on 4000 rpm for a while the vibration started, I pulled the clutch and let the revs drop and the vibration stopped.
then I let the clutch out and carried on at 4000 rpm until it did it again, this time I pulled the clutch in and kept the revs up and the vibration was still there.

I have been loosening the isos and head steady to see if that changes the vibration or stops it but it is still intermitantly there and hasn't changed from tighter isos.

Graeme

Hobot,
Chain and sprockets are new, no tight spots and adjusted with a small amount of slack. Enough to allow for suspension full movement.

If it were electrical I would have thought it would do it at an engine rev, 3800 and over, in any gear.

The feeling through the bike at 4000 rpm remains throughout the intermitant vibration, the intermitant vibe comes and goes while the engine etc feel the same.
I described the vibration as a rumble that is felt previously, but it doesn't do it all the time.
The vibration seems higher pitched than the engine speed.

Can a primary chain vibrate and set up a harmonic vibration that is mostly felt through the pegs, more than the bars and seat?

Possibly caused by a suspect auto chain tensioner?

How do I check this?

Am I suspecting this incorrectly?

Graeme

Hi there.
Have you checked rotor for loose center on crankshaft. just a thought !

GRM 450 said:

Possibly caused by a suspect auto chain tensioner?

How do I check this?

Am I suspecting this incorrectly?

Click to expand...

Suspect everything-until you find out what it is.

What type of oil is in the primary?

I know the hydraulic tensioner doesn't always work well with ATF. The recommended primary oil was 20w/50, which I've found is better for the tensioner and it doesn't appear to affect the clutch at all.

I've had primary chain set up a random isolastic-suspension wobble
of a few cycles per second, no noise just the forks tending to oscillate
to compensate. Only found solution when checking triplex
chain noticed it oscillated at the same freq. Then I found
out how loose-worn the gear box guts and swing arm bushes
were to allow slight chain hop to build up into whole bike
'osculation'. But it made no noise or rumble as you describe.

Closest I've had to your phenomena was with a rear bearing
coming apart, took cruise cycles of rolling before it'd line up at
worse then clear it for a while.

Two ways left to explore this, keep going till something
obvious develops, hopefully before it takes other stuff out
with it, or start from scratch and re-build everything.
Its entirely possible to re-build whole bike and not
discover what corrected it to run smooth after.

hobot

You say the primary chain and sprockets are original as far as you know, and have some 17,000 miles on them.

From what you describe, I do not believe your problem is with the iso settings, carbs, or electrcs.

Thinking about it, I keep coming back to one solution, and that is your primary.

If I were you, I would order in a new primary chain and front sprocket, and also a new clutch basket that has new sprocket for your primary chain,
Sounds like they are likely SHOT, no good, old, worn teeth, worn chain links.
Whether or not this is the ultimate solution, it needs to be done anyway because your stuff is old and worn out.
Deal with right now or deal with later when the problems can become worse and rip our primary apart with a chain, sprocket, total failure.

Without knowing more, I would also suspect that your clutch hub is all notched up from your clutch plates rattling in there. Your driven friction plates
are also nattered to stubs. So, may well also be time to order in a new clutch hub and plates.
I don't have a Mark 3 with an auto primary chain tensioner, but this may also be a part of the problem.
I think you need to open your wallet and order in just about everything that is inside your primary, sounds like it all is worn out.
This could solve our vibration problem immediately, and if it does not then you at least have replaced parts that you would need to replace very soon
anyway. And doing so would then eliminate a large step of the process to finding the ultimate solution.
This is painful and expensive I know as I rebuilding much of my own primary right now with new parts, but I really think you just gotta do it.

highdesert said:

Y
I think you need to open your wallet and order in just about everything that is inside your primary, sounds like it all is worn out.

Click to expand...

So easy to spend someone else's money, eh?

No Dave, I did not mean it like that. I meant nothing flip about spending other people's money.

But I did give my honest assessment, given his description of the originality of his primary parts, that in my opinion only it may well be the time to
accept that some money is going to have to be spent.

I apologize if you, or anyone else, took my comments as being demeaning, hurtful, or disrespectful of another person's financial ability to purchase parts.

Since you repeated what I said, I assume you felt I was out of line for suggesting it was time for some new parts, and that does mean spending money.

Maybe he can find some used good parts cheaper, got any ideas to save him some money?

Yep Commandos teach ain't no satisfactory half way measures,
either wait till an obvious injury reveals what to spend on
or start at scratch and renew and rebuild back to satisfaction.
This can add years to a nagging joy spoiler solution.
Proper sweet functioning example exist, for how ever long,
so we know its attainable, if fate allows and persistent to the end.

hobot

highdesert said:

No Dave, I did not mean it like that. I meant nothing flip about spending other people's money.

But I did give my honest assessment, given his description of the originality of his primary parts, that in my opinion only it may well be the time to
accept that some money is going to have to be spent.

I apologize if you, or anyone else, took my comments as being demeaning, hurtful, or disrespectful of another person's financial ability to purchase parts.

Since you repeated what I said, I assume you felt I was out of line for suggesting it was time for some new parts, and that does mean spending money.

Maybe he can find some used good parts cheaper, got any ideas to save him some money?

Click to expand...

I apologize, should have added a smilie face. You're probably right that best case scenario to replace all the parts, but that may not be the answer. Certainly we'd know more if we could inspect the parts in question (ahem, photos).

Kev-C, no I haven't had the chaincase off yet, but it looks like it will be comming off soon. Thanks for your thoughts.

L.A.B., I use 20w 60 Penrite in the engine and primary. And I do suspect everything, but I appreciate everyones ideas where to start looking.

Hobot, I have a voice in my head or a gut feeling pointing towards the primary chain, or a knackered bearing vibrating. But I'm not sure, thus I am asking advice before doing any un necessary dismantling.

Highdesert, How long do primary chains last before needing replacement?
The clutch friction plates plates are new Barnet and the steels have been checked. The clutch hub has very little wear and the clutch works very well, no slip or grab.
The money spent stopping this would be well spent, a vibration like this can't be good for anything, but where is it comming from?
I also realise that the drive sprocket with it's clutch gear, (for the electric start) the clutch drum and it's gear and the chain should be replaced as a unit.
I would love to be able to prove the primary chain is the culprit before forking out for new bits and finding it still does it.
I would prefer to replace parts with new rather than buying someone else's trouble. (false economy)

swooshdave, thanks for looking after my finances, but it's one of those things that comes with older bike ownership.

So how long are primary drive chains etc recomended to last ?

This about the only moving part that I haven't changed.

And I can take pictures but it all looked good when I put the cover on 5000 kms ago. But it didn't do the vibrate thing for the first 1500kms.
I will take the cover off and have a look.

Regards Graeme.

Hello. Just fixed a similar situation on a Mk3, turned out to be a loose center on the alternmator rotor. Cheers!

Stuart :wink: :roll:

Hello Stuart, that's what Kev-C suggested also.

Will check asap.

Regards Graeme