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MG MGB Technical - Vibration when reversing

I have a vibration problem with my 1965 mgb roadster which I can't get to the bottom of. When I reverse slowly ( below 1000 rpm )the car vibrates violently. It also tends to vibrate at low revs when going forward but not to the same extent. Last autumn the differential, half-shafts and bearings were all replaced by Brown and Gammons and, although this reduced the problem, it did not go away completely and gradually got worse again. When they MOT'd the car last week they checked it out again and tightened up all of the fittings but could find no obvious major problem. Tightening eased the problem temporarily, but after one week, it is back again.

Neither Brown & Gammons or I think that the problem is clutch related because once over 1500 rpm everything works fine and the gearchanges are as sweet as anything, but right now I am open to any suggestions. I cannot afford to keep spending money on parts & labour which do not solve the problem.

Can anyone help please ?
David Robinson

I am guessing you checked the engine and transmission mounts? Does your car have and OD unit installed? If it is malfunctioning and on in reverse it may cause strange vibrations. If you have one you can check by disconnecting it, but damage may have already been done. Did you also check the oil in the box for metal shavings? It sounds like you have had the car checked pretty well and if it's not any of these externel things I would guess something inside the box has either wore out or broke.
Also does the car sound like the engine is dying causing the vibrations like it wants to stall or does it seem like something in the drive train is hanging up?
J Arthurs

Does the vibration occur only while releasing the
clutch pedal? In this case the problem could
be clutch judder (oil on the clutch or just worn.)
A scarier possibility which has happend to other
posters is loose flywheel bolts. Also check for
a bad pilot (spigot) bushing.

The bellhousing to crossmember steady bracket (not
sure of the exact name of this) could be misadjusted.
If it is too loose or too tight you can get
judder-type symptoms. As J Arthurs points out,
check transmission and engine mounts, and also
the rubber mounts on the above-mentioned crossmember
bracket.
Ronald

I have a very similar problem - however, mine doesn't always happen. I've had new engine and gearbox mounts, etc...the only common thread I have on whether it happens on my car or not is how well the engine is running. If I've screwed the carb balance up, the juddder is worse, if the engine's cold, it's worse. Could it be that at low revs trying to engage the clutch make the engine run more unevenly and give the symptom of judder? Just a thought!

Tim.
Tim Jenner

If the carbs are out of tune/balance at a low rpm or when cold, one set of SU's will basically die before the other and cause the engine to go from a normal idle(maybe 60% of power thru one carb, 40% on other, to some extreme like trying to run 90% on one carb). If you remove air filters you can do a rough check by looking at the carb inner bowls. If one closes at low revs and the other is open you engine is really only running on two cylinders and those two are having to move the other two as dead weight. An air balancing tool will fix this and can be used for a more acurate check. There is a bolt on each carb where the throttle is. These can be loosened and you can then balance them so that there starting point, closed is even. Any good shop manual will have the procedure in detail.
J Arthurs

This thread was discussed between 22/06/2002 and 24/06/2002

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