Strange Vibrations...

As part of a full engine rebuild on my late '72 750, I installed Andover vernier isolastics and have been breaking the new rebuild in. The isos that came out of the bike had been there a long time, the front ones were particularly degraded, the rear less so but still worn out.

The new isos, combined with a CNW head-steady have tightened up the bike and it is truly a pleasure to ride. One thing that is new is an audible, high frequency vibration on the fuel tank at an indicated 2800 to 3200 RPM and it's most felt in second and third gears, often at 'around town' speeds. If I grip the tank with my knees, the vibration lessens but does not go away. The bike had the normal vibration just under 3000 RPM characteristic of the Commando before the new isos but not felt through the fuel tank like it is now.

The bike has an Interstate tank of unknown origin that I installed a few years ago with Commando exhaust rubbers as front mounts.

Has anyone else had an audible vibration through the fuel tank like this? I could try the stock stacked rubber washer front mount system but before I do, I figure I'd ask for advice...

Thanks in advance.

My guess is that something is hitting the tank and it acts like a sounding board. Try raising the tank slightly. Look for witness marks. Had this prob with my link rod headsteady.

As above, sounds like the head steady might be interfering with the tank. If this is the case, it needs sorting out as it‘ll rub through the tank otherwise...

I have also had vibes like this when the tank pads hardened and compressed and I had no tank strap installed.

Had the same vibration of the tank after iso replacement, it turned out that the rubbers were too hard, replaced the front iso rubbers again, (softer and narrower rubber) and the problem was gone. Many replacement iso rubbers supplied in the past were simply too hard.

I had the same once when I hadn't tightened the rear cross strap fully. Also If you have a braided oil line running across the head make sure that isn't touching the tank.

Cheers,

cliffa.

I replaced the iso's (front & rear) on my Mk III ES at the start of the year, they took a 400 miles to settle down. The bike seemed to vibrate more above 2.8k rpm, handling was tightened up & I was super conscious of the change & tweaked the adjustment. She was left for a couple of weeks (I was playing with the new Guzzi), when I jumped on the Commando everything felt 'just right'. so some of it was in my head.

I've had the interstate tank rattle when one or other front stud &/or the tank strap is not quite tight (I use a mix of rubber & steel washers) as per the parts book). When I installed braided rocker lines I added clear shrink wrap to minimise the chance of damage (e.g. my primary has wear marks [patina?]from the vibration of the earth lead to the starter motor).

My Dave Taylor style headsteady had the rose joints knocking against the tank tunnel. Tank is a re-pop, so not sure if dimensional differences are the real culprit. Used a block of 2X4 lumber, a 1/2" carriage bolt with domed head, and a nut & washer to make a tool for creating some relief bends near the rose joint witness marks. 2X4 had a 1/2" hole drilled near mid point of it's 8" length, such that bolt end could fit easily in. Bolt cut down to needed length to span the tank tunnel and some extra length in the drilled hole. Nut & washer used to thread against wood, pushing bolt away from wood into far side of tunnel to make the needed relief bend. Flipped all around and did other side of tunnel. Worked very well.

my experience same as tornado emgo tank/taylor headsteady I took Dremel to head steady and gave microscopic haircut on rose joints to clear tank by C#ckhair all good now

Thanks for everyone's insight.

I had the tank off, no problems with the head steady or anything else contacting the tank. The front of the tank was mounted on exhaust rubbers and I replaced those with the proper stud and four stacked rubber washers on each side. I also noticed the rear cross mount was barely touching the frame tube when tightened so I added a piece of rubber to it so it was actually holding the tank down when tight.

The vibration is gone now - there is still the 2600-3000 rpm slight vibe but not felt through the tank at all.

northern750 said:

Thanks for everyone's insight.

I had the tank off, no problems with the head steady or anything else contacting the tank. The front of the tank was mounted on exhaust rubbers and I replaced those with the proper stud and four stacked rubber washers on each side. I also noticed the rear cross mount was barely touching the frame tube when tightened so I added a piece of rubber to it so it was actually holding the tank down when tight.

The vibration is gone now - there is still the 2600-3000 rpm slight vibe but not felt through the tank at all.

Click to expand...

I found exhaust rubbers on the front of tank did not provide enough length to protrude down through the frame brackets to get nuts on. Maybe an issue with the re-pop tank or maybe my fresh foam in tank tunnel too thick/not placed correctly. Studs and rubber disks (4 above, one below the frame bracket) work ok. I do like the rubbers on the rear mount points in place of studs.