Hi,
just happened to be looking at the wiring diagrams, stock the cars had a 1.4 ohm resistor wire or I here that some had a Ballast resistor.
There is also a wire coming directly from the starter relay for a hot spark during startup and this may be were spike is coming from.
but I must say I have never noticed my tach jumping as you describe, just never paid it any mind, will be looking now though.
Mark
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