I have a new cable in my 1970 tach, but the needle bounces around most of the time and smoothes out here and there but still has a little movement. Fairlane5142010-12-23 22:44:05

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The tach itself bouncing will have no effect on your ignition timing at all. From the sound of it, the problem is most likely gonna be the tach itself. If you have a reversable drill, you could spin the tach cable with it, and see if the noise is still there, and the needle still jumps. If it does, then your issue is with the tach. If it does not, you prolly need to remove the tach driven gear from the distributor and have a look-see. The fitting on the dist where the cable connects is screwed in, and can be removed. IF the gear there is damaged, chances are high that the dist shaft itself is also damaged...

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Joel Adams
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Did you put a little lube on the new cable before you put it in?  There is a chance that it is sticking and jumping.  Otherwise, I completely agree with Joel.

Larry

I had the same prb in my 69 350, only 15K miles on it and the tach dropped to 0. Replaced the stripped tach gear and it started working, but jumping and then it quit again. It turned out to be the distributor shaft having a broken tooth, so rather than replace the dist I added an electronic tach on the steering column. Then you can set your own redline!

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Joe T