HEI started in 1975 in Corvettes, and they had a "tach filter" between the distributor and the tach.
I installed a Pertronix setup in my '68 Zed a couple of years ago and didn't notice if it was a corresponding problem time-wise, but my tach (Sun Super Tach II) started pulsating on a non regular basis about or close to the time of the Pertronix installation. I read to check for wires being in proximity of a plug wire or similar logical stuff that might cause such a problem - but nothing is obvious or has been found to cure the problem.
Would or can a Pertronix unit (its the newer version - sorry the exact model name escapes me right now) cause the same type of problems as a HEI installation or something similar to this?
My tach seems to read in the correct RPM range it should when at idle or driving - it just twitches up the scale every secong or two like its getting a signal from somewhere jolting it... or maybe it just needs replacement or a get a rebuild if that is possible somewhere.
Sorry of this sort of highjacks this thread but it seemed somewhat related...
Thanks - Randy