Here's an odd one.... I recently added a Pertronix Ignitor I system to my 327 Chevy powered car. A friend and I were making modifications to improve performance. He suggested that replace my modern resistor plugs with AC Delco non-resistor plugs. I happen to have an old set. We stuck them in and the car seemed to run quite good. We then noticed that the Tachometer was reading about 2x normal. It is as functioning properly even after the Pertronix system install. I checked the new Flamethrower coil. All connections seemed good. I thought it was the usual compatibility issue found with old Tachometers. Today I decided to change back to resistor plugs. It was the last change made before blaming the Pertronix system. Immediately after that the Tach functioned perfectly. I was very happy! So it seems that the Pertronix system doesn't not like resistor plugs. At least with a tach made in 1962. Sent from my Pixel 3a XL using The H.A.M.B. mobile app
Um...I ran a Pertronix (first design) in my old Stude engine, with "non-resistor" plugs for about 10 years. I ran Autolite or Champion plugs over the years. My Autometer tach worked just fine..! Mike
Ok. It may not be true for everyone. I also using the Flamethrower Mag x2 plug wires. I think they have a spiral wound core. Sent from my Pixel 3a XL using The H.A.M.B. mobile app
The resistor plugs (like carbon string plug leads) are supposed to limit the back EMF -i.e. a big pulse that goes (as a result of the spark) from the plug back to the coil secondary. This pulse is then induced into the primary and is likely to fry some electronic ignition setups. It sounds like the OP's tacho feed is picking up the back EMF as well as the real pulse which results in a doubling of the reading.
That explanation makes good sense. Thanks for letting me know why it happened. Sent from my Pixel 3a XL using The H.A.M.B. mobile app
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