If it's got its power connected correctly and the input wire connected to the (-) coil contact (for points ignition), or the "TAC" terminal (for HEI), than I'm guessing the tach itself. Have you had it refurbushed??? The originals are notorious for giving up the ghost. The tracings on the circuit board pull away and the spring tensioners on the needle get out of whack.
I have been helping Dave with the inst. cluster install. The 12 V terminal is hooked to the factory 12 V tach wire and the Coil terminal is hooked to the MSD tach wire. The tach was refurbished and Dave said it was converted for use with an MSD ign.
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