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Bike: 1977 R100S 46,000kms

Vibration felt mainly thru footpegs begins around 4200rpm and increases as rpm's increase.

Thought was related to loose pinion collar nut but for past few weeks substituded another FD from a 78 R100/7 to get bike back on the road for some riding and sorting out things after the S,s rebuild this past winter...collar nut on borrowed FD was check and on real tight. Vibes still there.

Things checked so far:
Engine mounting bolts checked - all ok
Rechecked timing - spot on
Rechecked valve clearances - all ok
Re-synced carb balance at both idle and just off idle - real good using 2 different manmometers and also my buddies. Also balanced at around 4000 rpm's...very close on meters...not enough to make a difference
Rechecked new throttle cables installed at rebuild to check for routing and/or any hang ups - didn't find anything so far

With bike on centre stand...free hand on footpeg...run rpm's up to 4000...not bad but hit 4200+ vibes start and continue right up to 7000 (didn't go beyond that)

So...don't think carb balance is the root cause...now think its either flywheel/clutch, gearbox input shaft (?) etc....

Any ideas where to look to now? Vibes strictly related to engine/gearbox combination as it happens in neutral, on the center stand.

Add'l Note: Searching through old work orders PO gave me with bike. In 1994 bike went in to "check possible slipping clutch" but I didn't see anything that indicated clutch was replaced or checked. Wondering if they had clutch/flywheel out and its now out of balance if reinstalled out of balance???

Thinking to remove gear box and run engine only to see if vibes are still there...would eliminate gearbox input shaft ????

Running the engine without the tranny and clutch may give an indication.

Pity you can't run the tranny and clutch without the engine.

I think 4200 is a common rpm for /7 vibrations to begin. I think.

That is a common rpm range for some vibration, but the band should only be 300-400 rpm wide and smooth out very well.

You might check these items.
1. Differential timing
2. Carb balance at that rpm, not at idle.
3. Compression.
and other things that I forgot.

A very small leak in one carb diaphragm might manifest itself in that way.

A borderline coil might also behave that way.

Good ideas for other possible things to look at...thanks

Did do carb sync at around 4000 rpm using 2 different monmometers...darn close as we could get

Compression check...can see what it is

Diaphragms...hmm...will check that out. Never replaced in carb rebuild as they looked good in inspection but have a new pair on the bench I'll pop in to see.

Differential timing...don't know what I should be looking for/ checking...can you elaborate

I know therre is always a vibration around this rpm level, but, as Duane ponts out, they all smooth out 2-300rpm later. But this was continues on right up to 7000..

I'm going to check the easy things first that have been pointed out and see where we go from there. I still believe it something simple...and hoping not imbalanced flywheel/clutch. The bike runs beautifullty up to 4200rpm about 120kph...

Moose, If your '77 carbs never had new diaphragms, you will be surprised how much better things will work if you change them. You might like what new coils do for you, too!
Vibes. Loosen the engine mounting-stud nuts and tighten them again. Do not just tighten things without loosening A LITTLE at first. Do the same thing with the fasteners that hold the tranny to the engine, and the flange on the driveshaft. Check that the subframe (The rear frame) is fastened securely and not in tension...Also footpegs, etc.
The 4000-4200rpm vibration is SOOOOO normal, but it should not persist, and it can be minimized. Something somewhere is loose or stressed-in-assembly.
Do you feel the SAME vibration in 3rd or 4th gear too? If not, that would help eliminate some of the possible suspects related to the final drive, gearbox, and wheels...

Duane Ausherman wrote:That is a common rpm range for some vibration, but the band should only be 300-400 rpm wide and smooth out very well.

You might check these items.
1. Differential timing
2. Carb balance at that rpm, not at idle.
3. Compression.
and other things that I forgot.

If I only just new "the other things" Duane has forgotten, I'd be very pleased with that...

[quote="moosehead"]

Did do carb sync at around 4000 rpm using 2 different monmometers...darn close as we could get

Compression check...can see what it is[quote]

I know that it seems logical that you can balance carbs with a manometer at road rpm, but it isn't the same. I would test it by riding it and adjusting as I went.

If the diaphragms and coils are original, I probably would start with replacing them.

Please get back to us when you find it.

Here is an odd ball vibe that happened to me.

the cushdrive spring on the d/shaft broke. This allow the ramps in the assy to climb up & down the slopes causing a vibration at about 2500 to 3000 . I checked EVERYTHING at least three times before I found it

Forgot the First question I should ALWAYS ask:
Did it always do this, and if not, WHEN (what did you DO?) did it start?
From your first post it sounds like you disassembled the engine and tranny.