MG Car Repair: 1977 mg midget vibration, wheel wobble, engine mountings

Question
QUESTION: my 77 mg midget 1500 has a bad vibration on acceleration and deceleration at about 60 mph.it was bad at 45 mph until i put a new propshaft on it.the wheels were all balanced.i was told by a mechanic friend it could be the rear gearbox mountings but i,m not convinced as they don't look too bad and its an engine and gearbox out job to replace them. any ideas?

ANSWER: HI Dave.  I presume you can feel the vibration through the steering wheel?  Wheel wobble at about 60mph is the classic symptom of front wheel imbalance.  If you have wire wheels, they have probably been balanced on the wrong machine.  You need special alignment cones to do the job properly, and most garages do not have them.  If you can drive through the imbalance and it goes away at 70mph that would be conclusive.  Soft or perished gearbox mounting rubbers could make a vibration worse, but they would not CAUSE a vibration.     

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QUESTION: Thanks for the prompt answer but its not really through the steering its the whole car that vibrates especially when decelerating, which is why i changed the prop.I have removed the steady bracket on the back end of the box(only fitted for about a year on 1500's) as i was told the rear of the gearbox was resting on it, but this doesn't seem to have solved it.they are rostyle wheels not wires by the way.

Answer
Hi Dave.  Since the whole car shakes, then probably one or more of the engine mountings has failed, but I am concerned that you also have something out of balance, or possibly the propshaft front UJ is touching the transmission tunnel due to soft or failed mountings.  

Since this happens on deceleration, I would suspect the nearside (left front) engine mounting, which takes the load as you decelerate.

1) Can you access all the mounting rubbers and prod them with a screwdriver?  If the rubber is hard, that is a good sign.  If the rubber is soft or covered in oil, that is a bad sign.

2) Find a piece of plywood, put it under the sump pan to spread the load, then lift the engine & gearbox with a jack.  You will be able to see if one of the mountings has come un-bonded.

Here is a diagram of the components:

http://www.moss-europe.co.uk/Shop/ViewProducts.aspx?PlateIndexID=1374