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September 29th, 2008

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Q & A: 1980 MGB Vibration

Monday January 02nd, 2006
Q & A, MGA, MGB, MGC, MG T-Type, Midget
John...

Tony Lane and Tom Piacenza attended your transmission tech class last week and may have mentioned my dilemma to you. I am running a 1980 MGB that has been one of the tightest, smoothest MGs that I have had the pleasure to own. I know you are not keen on the Weber downdraft carb, but I am running that conversion with the early pre-Zenith exhaust manifold.

My problem is a strange vibration that begins around 2300 RPM and peaks at 3500. It is most noticeable at cruise and is on the verge of violent when backing off the power at those revs in high gear. It reminds me of a back pressure vibration that resonates front to aft throughout the car but mainly in the exhaust. It can get really boomey when getting off the power from speeds above 55 MPH. The vibration disappears when the clutch is disengaged which narrows it to engine/driveline.

Last Fall, Tom Piacenza and I pulled the engine and gearbox to replace sloppy motor mounts and related broken brackets. We did use Moss's heavy duty motor mounts. No big deal. We have done this procedure a dozen times. We also replaced the clutch assy with Borg & Beck pieces, gearbox input and output seals, rear main seal, timing cover seal, and pinion seal. The only odd thing that we encountered was a crack in the down pipe flange that snugs up to the manifold. Tom welded this ring for additional support and I was a little concerned about clearance with the donut gaskets in place. Also, the downpipe is a tight fit as I don't believe the new studs on the manifold are exactly in alignment. At any rate, we carefully reassembled everything and encountered the weird vibration.

On advise from your tech notes, we focused on the Ansa exhaust and decided to replace it with a new stock system from Moss. I also pulled the driveshaft and had a local driveline specialist install new U joints and balance the shaft. The problem still exists to my frustration and amazement. Do you have any ideas or can you suggest a likely culprit?

Safety Fast!

Tom
Tom!

First test: Sit in the driveway and rev the engine up to the rpm where the problem occurs. If there is a vibration, then the problem lies in the exhaust.

If the problem ONLY exists on the road (which I suspe... read more