problem finally solved

I have been working on a 1971 Norton for quite a while now. For months, off and on, I have been trying to solve a vicious vibration problem mostly felt from 2800 rpm on down. Above 3000 it was tolerable but still nowhere near what a Norton should be. The top end is new, the isolastics are new and adjusted to .010. Single 34 Mikuni. Despite every effort I couldn't get the shake out of it.
I finally figured it out. The mufflers provided by the owner were the culprit. They looked identical to the standard reverse cone mufflers but they didn't sound the same. Lacking any other ideas, I switched them with known good mufflers and cut the shake down by at least 50% or more. And narrowed the range down to about 200 rpm, perhaps 2300 - 2500 rpm. About 3000 the motor smooths out like a Norton should.
These mufflers are about 6 years old but new so I didn't suspect them until I exhausted all other possibilities. Has this happened to anybody else? Anybody have a mysterious tuning problem that can't be solved? Try another set of mufflers.

Hi Seattle,
Do you know the manufacturer of the culprit mufflers? Would be good info on what to avoid.

Ed

This is a first for me, but perhaps total weight is setting up a harmonic that goes away with different weight parts. I'd be curious to see a weight comparison between the two systems.

You "exhausted" all other possibilities? There's a clever lad.

Different brands of mufflers have different back pressure, which greatly affects engine performance. Your engine was struggling to breathe! (If you've ever had a stuffy nose, you know that can be VERY exhausting)

I don't know who made the mufflers. They really look like stock peashooters. Short of cutting them apart, I will never know how the construction differs. Never before have I had mufflers make such a big difference for the worse. Took me a long time to find the problem. I almost gave up on it.

At 3000 RPM, or less, I would think that the mufflers wouldn't have (much of) an influence, being a much larger bore than required at such low RPM. Your experience, obviously, belays that.

However:

The symptoms you mentioned have me wondering about jetting and compression, maybe about cam timing (if it was messed with) and ignition (if you have twin points). Any history here?

I'm wondering if you can see "straight thru" the culprit mufflers? I had a "new" aftermarket set years ago, that the bike ran terrible with. Looking thru the mufflers and you could NOT see the thru! Who knows what kind of "maze" the exhaust gases had to go thru; and they "looked like", peashooters. I then installed OEM Norton peashooters that you could see thru, that just had flutes in them. Bike ran great then.

i'd had various combo's of single pistion running, mufflers falling off, internal baffle tubes breaking up, headers cracking apart and mounts rubbers failing plus a 2-1-megaphone - w/o effecting the isolastion rpm, enough so only realized half an engine by the delay to get to hwy speed again. So I'd susspect the prior muffler mounting put too much tension on the Z plates so transmitted vibes. I've learned my lession to not clamp muffler on headers itght enough to restrict some sliding action. If anything on power unit touches frame - you will not be able to stand the vibes, feels like thrown rod - till found and fixed, so don't suspect anything touching in your case.