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MG MGB Technical - Bouncing speedometer

The post on the inaccurate speedometer brings to mind the fact that mine bounces around over about a 2 to 3 mph range.

This is on my 73B. The tach needle sits and moves perfectly evenly but the speedo needle bounces around. It occurs at all speeds.

Is this to be expected or do I have "something else" to fix!?

Thanks,
BH
BH Davis

Sometimes a bounce in the speedo reading will be due to a problem with the speedo drive cable. Most commonly it will be that the gearbox end is too tightly bent. It is often followed by a failure where the square section part joins the end of the cable
David Witham

I just replaced the cable on my car and cured a 5mph bounce! The tacho is electronic and so is not linked in any way to the speedo.

Iain
67 BGT
I D Cameron

Trace the path of the cable and make it as straight as possible with the longest gentle bends/curves as possible.
The bad news is that once you get bouncing, caused by the cable being slightly bent, it is very difficult to cure completly without buying a new cable. If you have ever tried to straighten out a piece of bent wire you will know why.
I tried lubricating the cable with graphite, don't, it built up into lumps and made things worse.
Some people fit right angles drives to the cable to keep it straight as possible. This is a good idea, but expensive.
As long as it's not bouncing too much the best idea might be to consider it part of the charactor of the car!
Peter

Does it bounce in time with the odometer movement? Or several times faster?

If faster it is almost certainly a cable problem - either dry or broken strands digging in to the outer momentarily stopping the inner.

If the same, on mine it was due to a dry speedo head. A drop of oil on the input shaft solved the problem even though some say oil here is death to speedos (they are lubricated internally so I don't see that). A dry cable could also cause this, where the additional load of the odometer (more noticeable on big number changes) causes the inner to wind up a bit.
Paul Hunt 2

To check if the drive cable is faulty connect the speedometer to the rev counter cable.Then run the engine up and hold at given indicated speeds (40 % 60)If the needle is steady then the cable is faulty. If it does wobble check the following

clearance inside the speedometer for the drive cable. If there is no clearance the pressure on the bearing can cause trouble. Shim with washers accordingly.

Oil as already suggested.

Clean the drive cable by removing the inner flexible element and pull through the outer casing cloth or similar until it comes clean. Clean the inner flex and very lightly grease with Castrol LM.

If the needle still flicks about the problem lies either with the bearing in the speedo or the magnetism has been upset. Both reqiuore specilaist skills to rectify.
Bob Marshall

I had a bouncing speedo - bottom line was oil coming up from the gearbox contaminating the head. It is not a simple thing to clean the inside of your speedo so if you do it ... rather you than me. However, also check for the presence of a felt washer in the lower drive connection - this should soak up oil and prevent this from happening. The washer may not be there, worn out,or saturated - either way, allowing oil to pass though to the head.

Paul
Paul Barrow

This thread was discussed between 21/06/2007 and 29/06/2007

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