MG Car Repair: MGB Speedometer, speedo cables, thumb nut

Question
Howard

I recently installed an overdrive transmission in my 71 MGB GT.  All went well.  I had done this once before with my 72 Roadster.  The only problem with both installations regards the speedometer.  While it worked fine with the old tranny and cable, with the new set up, I get significant needle swing and I think it is reading high when you can get a reading.  I replaced the non OD cable with a new OD cable from VB for both cars.  On the first car I just assumed a cable routing problem and learned to live with it.  On the second car I resolved to fix it.  I routed the cable similarly to the original and it still jumps.  I even went so far as to put the car on jack stands, route the cable to the engine compartment and attach the speedo there so the cable has only one bend and it still jumps when the wheels are turning.  I'm wondering if the cable itself could be the source of the problem.  When I watch it turn, It looks like it may be eccentric in nature whereas the original is very circular and smooth.  I did the previous conversion about 5 years ago but also used a VB cable.  Have you had any problems with aftermarket speedo cables?  I know intuition says the cable must be kinked or binding but my right angle adapters at both ends seem to be working smoothly and with only one gentle bend in the cable I would think it would be fine.  My next step may be to lay the speedo on the ground and route the cable straight to it to see if it still jumps.  Any ideas?

Dave

Answer
Hi Dave,
I found it a good policy to put a short piece of old cable into the drive and rotate the driveshaft to confirm no slippage in the drive gear by holding tension on the cable between my fingers. Another possible problem is that the cable may be a little long and applying pressure to the end of the speedometer shaft. To test this I would just remove the speedo and hand hold it and install the cable and housing but leave the thumb nut loose and pull straight out on the housing while on stands and running in gear to see that there is not a problem in that area. Also, did you lube the cable when you got it? Many come dry from the manufacture.
Howard