Mk3 Vibration

Hello, Firstly, I have never owned a Commando before this one so I'm not sure what they are supposed to vibrate like.
I have another topic about "strange vibration" refering to an intermitant vibration at 3800 rpm and over.

Anyway, I tried adjusting the isolastic mounts looser and the head steady also looser to see what that does.
Now with the isos looser the whole bike vibrates up to 3000 rpm then smooths out, (but the strange vibration is still there at 3800 and over)

Should a Mk3 vibrate heavily throughout the whole bike with the isolastics and head steady loosened?

From what I've read here it should smooth out about 2500 rpm.

If I tighten everything and and back the isolastics off a bit as per the instructions the normal engine vibration disapears at 2000 rpm and is smooth through out the rev range. (strange vibe is still there intermitantly)
Why does this bike vibrate worse with the isos loosened?

I can move the back wheel about 3/8 ths of an inch with the isolastics loosened, to give some idea of how loose they are.

Graeme.

GRM 450 said:

Should a Mk3 vibrate heavily throughout the whole bike with the isolastics and head steady loosened?

From what I've read here it should smooth out about 2500 rpm.

If I tighten everything and and back the isolastics off a bit as per the instructions the normal engine vibration disapears at 2000 rpm and is smooth through out the rev range. (strange vibe is still there intermitantly)
Why does this bike vibrate worse with the isos loosened?

Graeme.

Click to expand...

Dunno what you are trying to prove other than the factory figured this out back in 1967. The iso rubber dimensions and assembly works within the balance factor of the motor in its rev range where the vibration increases and decreases. By loosening all of the above mentioned, you have an unrestrained motor bouncing around in the frame. If you tighten up the isos under .005" you will find vibration increasing again. The sweet spot is .010".

A critical factor is setting up the top mount, done wrong that can transmit vibration. But you have already proven as per your comment above that by setting the isos by the manual it is smooth from 2000RPM onwards, so there is nothing wrong with your bike. But don't run around with excessively loose isos and that much play in the swing arm as your greatest risk is loosing steering control.

Mick

Graeme, hope this isn't last Commando that makes you swear off them.
Very good sign that normal iso gap reduces the silent threshold under 2300.
If you only get intervals of vibration intruding I'd suspect something messing with the combustion on one side or the other intermittently. Resonating poor connections in terminals on inside insulation can do this. Look after dark and also a test
light or plug exposed with key on and jiggle tug wires while listening for a spark
then narrow down.

Sometimes can be drive train wear slack with clutch basket and primary
chain oscillations playing on the rear chain feeding back into isolastics
at front to bind in waves.

I've had good success getting my 2 Combats silent, but took me some weeks
to months in intervals of annoying mystery. Still not sure I understand
full scope of what makes Commando such flying carpets to chatter boxes.

hobot

Thanks Mick and Hobot,

Mick, the only reason I loosened everything was to see if the intermitant vibe was removed or changed, it didn't. So I will reset them to 10 thou.
The head steady, I've set this as per the manual with the measurement stated, but I assumed the spring would have weakened during 34 years of being under tension?????
Is there a sure way of setting this tension? or is it trial and error? or just set it as per the manuals distance settings?

Hobot, I understand what you are saying about and electrical misfire to one cylinder causing the engine to fire unevenly, but it runs sweet enough? although I think it feels underpowered for the engine size, especially above 4000 rpm.
I've checked the timing (pazon) and advanced about 3 degrees with no improvement in higher rev range performance. (everything engine wise is standard, except 34mm Mikuni)
This bike isn't for fast riding, (I have other bikes for that) but I feel it should go better than it does. It is a very good cruiser, and that is not a problem to me.
I have a feeling that the "strange intermitant" vibration does have something to do with the primary, but I can't see anything at the present. (it will probably show itself in the future)

graeme

Ugh, hope what ever it is, shows up close to home.
If an 850 the few stock ones I've ridden sorta tease you with initial torque but don't keep reving and pulling like a good ole 750.

Commando seem to tighten up and smooth out under load and power but
can feel weavey to clattery when loads get relaxed and slack shows up.
Worn tires, air pressure balance to sagging aged iso doughnuts.
Just keep in mind that iso's can take up lots of vertical oscillation but
hardly any at all of horizontal imbalances.

I'm taking my time to get a Combat engine back in, to delay facing isolastic
dial in tedium in a skewed by deer strike frame. It worked fined
but for these intermittent buzzes in some leans. Fixed by coil connection.

hobot

Graeme -

I have a Mark 3 and it is as smooth as butter - and at low RPM, probably less than my BMW R 1100 RT.

Maybe it would help us remote diagnosticians if you could describe this intermittent vibration? Is it felt in the handlebars? the pegs? any relationship to wobble?

Does it come on acceleration? deceleration? On entering a turn? exiting?

Any sounds associate with it?

Keith

Hobot,Your explanation of being teased by the 850 low down torque and then dissapointed by the upper end is spot on. Similar aged and size Ducatis leave it once over 60 mph, and my 450 single revs much more freely. But not having riden another 850 Mk3 I don't know what they are suposed to go like.
I had the frame checked and straightened before the rebuild, and all went together ok.
New iso's and wheel bearings etc, etc.

Keith,
The intermittant vibe is mostly felt through the foot pegs, but the bars and seat as well.
It starts like a switch being turned on at 3800 rpm (approx) and only after about 10 kms when the oil is up to temp. But with loose iso's it does it straight away (above 3800). It seems to only do it it 4th gear.
It does it on a cruising throttle at 100 kms (60 mph) and comes in suddenly and stays there if the speed is increased, and I have to slow to 3000 rpm for it to stop. (suddenly again)
It doesn't seem to do it in any other gear.
If I'm having mountain road fun I don't notice it at all, only boring highway cruising.
Changing the bikes upright straight line stance to leaning left or right doesn't stop the vibe, or buzz as Hobot refered to it.
When I first noticed it, I described it as a rumble, but now it's more a vibration buzz. The frequency doesn't seem to change if the engine speed is increased.
The only way to stop it slow below 3000 rpm.
??????? I know it will show itself if it keeps doing it, but how is the scary question???

I've checked the primary chain and auto adjuster, and changed the primary oil to a lighter grade,
Wheel bearings, tyres, drive chain and sprockets,
Nothing is loose, nothing is touching the frame, everything is tight.

graeme

Graeme,
Is the suspensary ( how do you spell that ) spring thingy too tight ? If so, it can have an effect right through the rev range. It's there to remove the low frequency vibes at 2500 to 3000. But, if over adjusted it can cause the effects you are feeling. ie the low frequency vibes are gone but coz the ISO is now hard up you get a buzz higher up in the rev range. Try backing it right off and if that works tighten in small increments until the low frequency vibes are tamed.



Best of luck,

Cash

You should be getting plenty of power at 4000 rpm with a single 34mm Mikuni. I found that it worked very well there - the front end would get light and just dance on the surface of the road under accel in 2nd gear with my combat. I got rid of the single Mikuni because it ran out of steam on the very top end. Also I tried dual Mikunis but that hurt midrange. The Amals worked great everywhere so I am back to them (resleeved).

I don't think it would be very different for your 850 but those bikes feel entirely foreign (like a tank) to me.

My meager experience exactly agrees with batrider's on Combat vs 850's
and carbs, but find bigger 34mm could pull a bit higher top end but was more
sluggish in more sane thrill and traffic flight envelopes.

You should be getting plenty of power at 4000 rpm with a single 34mm Mikuni. I found that it worked very well there - the front end would get light and just dance on the surface of the road under accel in 2nd gear with my combat. I got rid of the single Mikuni because it ran out of steam on the very top end. Also I tried dual Mikunis but that hurt midrange. The Amals worked great everywhere so I am back to them (resleeved).
I don't think it would be very different for your 850 but those bikes feel entirely foreign (like a tank) to me.

Click to expand...

Spring setting info is useful to me as Peel new engine is heavier than 750 so have spring to adapt and figure out if helps anything or not.

I still vote to suspect electrical grimlin, but anything can cause anything on Commandos so let us know what you do and what happens.

hobot

My Bike has the lightweight piston of course - so its smooth. But I also have a phenomenon that comes in at cruising speed in 4th. At first I thought it was some kind of harmonics in the pushrods or something weird in the motor. Turns out that it was rear chain slap. Looking down I could see the chain go into a flutter as if I was riding a big single that was pulsating the chain. The top 1/2 of the chain was hitting the frame on my featherbed (protected by a plastic guard). The slap was making a ringing noise and creating slight vibration and was bugging the shit out of me until I figured it out. I changed gearing but the problem is still there. Its not really so bad and I just ignore it now that I know what it is.

Take a ride and look down at your rear chain when the vibration comes in. A clue is somewhere where you said "The frequency doesn't seem to change if the engine speed is increased." Thats the way mine was behaving.

Jim

Jim,
I will definatly have a look at the rear chain, I've never had that happen even on a single with a nackered chain.
At this point I'll try everything, thank you.
graeme

If swing arm slack and/or gearbox sleeve bushes slack then both primary chain flop and swing arm slop can tug on drive chain to jump rope which tugs back to front mount and slaps the forks to wiggle. Usually sensed more as a handling issue than a vibration one. But anything can cause anything in Commandos.

If Commandos weren't such smooth operators who ride em much and market would drop out to support them and that would of been the end of em a while ago.
If it ain't smooth in mid 2000's then its not yet ripened to a matured Commando.

hobot