Well, I had the same problem with a '77B. Turned out to be a
gear inside the speedo head.
1. Get under the car (I assume you will jack it up first, or are
really skinny) and unhook the speedo cable from the tranny. Turn
the cable insert by hand and see if it turns freely and smoothly.
If it does, you have some other problem.
2. If it does not turn freely, remove the speedo from the dash.
Put a short peice of cable in and turn it and see if it still
does not turn smoothly and freely. If it does, replace your
cable.
3. If you are brave, open it up. If you are not brave send it out
for rebuild or replace it. I found the nylon gear driven byt he
helical input gear was cracked. So instead of replacing it (which
I will do some day) I filed the teeth so it would work smoothly.
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