Vibration at speed; R90/6

My son bought a '74 R90/6 last winter. I am just about done with the restoration/road prep, etc. In fact, I've posted many questions pertaining to this bike and my own R90/6 in the last few months.

Overall, the bike is getting dialed in. It starts and runs very well and is in a high state of tune. I've completed the spline lube and the transmission shifts smoothly and feels like silk. The suspension is its strong point though, it feels like riding a couch on rails.

However:

There is a resonant frequency vibration at 3000 rpm and also at highway speeds of 4-5000 rpm it feels just a bit too "buzzy." Motor mounts are tight and solid (I'm going to double check). Carb sync is spot on. Although at highway speeds that wouldn't matter so much. Compression is good on both cylinders. It does smoke on start up after sitting on its sidestand and sometimes even when on the centerstand.

I must also say that it has a sort of dull clatter coming from the front of the engine. I assume this to be a timing chain issue. It doesn't sound like it is anywhere close to failing however. It is a noise that my R90/6 doesn't have though. Mileage is around 55,000. I'm trying to talk him into a top end rebuild this coming winter simply because it is easy to do.

I would like to resolve this problem past the standard solution of getting "Mushy" grips and footpegs. Picture below;

From time to time when people change their clutches, there is the mention of marking the parts for reassembly. I wonder if possibly a PO had the clutch apart and it didn't go back together the same way. It would be a "grand experiment" to begin rotating the various pieces relative to each other and see if one combination is the best.

When I did the spline lube I did not see any indication that anyone had ever been in there before. However that does make a lot of sense. I wonder if there is a shop that can balance the Flywheel/clutch assembly? I would think that disassembling everything and then taking it out for a ride several times would be a lot of work. Seems simpler to send it all off for balancing.

Check with Southland clutch...

http://www.southlandclutch.com

They refurbish clutch discs and may do a total balancing.

godfather said:

Check with Southland clutch...

http://www.southlandclutch.com

They refurbish clutch discs and may do a total balancing.

Click to expand...

Just got off the phone with the guy from Southland clutch. The guy is a genius! More importantly, he is extremely helpful. I can see why you would recommend him. Unfortunately, he could not help me. Here is why: He stated that with later model airheads and newer oilheads, etc., balance is critical and he recommends to ALL that they index their clutch components before dis-assembly. However with the earlier model BMW's such as mine, because the flywheel is so large and heavy they NEVER see a balance problem. In fact, he stated that focusing on the flywheel/clutch balance without taking into account other spinning mass such as the crank could compound the problem considerably.

He asked me to describe the problem and suggested I look to a source other than flywheel/clutch. There is another thread on the board about a Seibenrock kit. I described how before installation of the kit I had a 3000 rpm resonant frequency vibration......JUST like I do on this bike. The only difference being is that mine vibrated a little less at touring speeds.

I think that the timing chain is a little noisy, not catastrophic failure noisy, but louder than my other R90/6. Has anyone ever heard of balance problems associated with timing chains?