Tach bouncing
 

Have a 61 Safari that was previously converted to alternator and PERTRONIX ignition. The Moon tach in the car was very lazy and did not read anything above 1k most of the time but it didnt bounce. Just replaced cam and needed something accurate so bought a cheap chinese tach for break in. It bounced but I was able to get my cam broken in. I exchanged that junk for another one exactly the same. This one bounces pretty bad too . Tach has a shift light thats wonky too. I am thinking at least one out of two would work but maybe not ! I have 14+ at the alternator running. 13.7 at the battery and 13.5 at the coil + terminal. Its driving me nuts but dont want to destroy a good tach if I replace it. Could it be the coil or Pertronix ? Both are 12 years old.. took a video but cant post it.

I do not have much for personal experience on this, but some ignition systems require a tach adapter module available at MSD.

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Great suggestion thanks. The MSD wont work but Classic Instruments makes a filter for this system. Apparently noise in the signal so lets see what happens !

Have you got a radio capacitor hooked to coil(+) terminal?

If so, try unhooking it and see if it helps.

Resistance plug/coil wires going bad will cause arcing at the coil and make a tach act up.

With point distributors, a bad condenser and weak or broken breaker plate ground wire will make a tach act up.

Clay

No radio capacitor but the wires may make sense. I will have a look later tonight when it gets dark ! Thanks. The wires on the car are at least 10 year old Taylor wires. Worth a look for sure.

Well I installed a filter and it stopped the bouncing at anything over 1k rpm. Once I let it drop to idle which is 800-900 it bounces. I can kind of feel some slight misfiring down below 1k . I see no spark leakage from the wires. Tried setting tach to 4-6-8 cylinder but 8 is where it needs to be. Everything else checks out. Voltage to coil , air gap between magnets and pickup. Haven’t checked coil resistance yet but going out later to check. I reached out to Pertronix. Maybe they have an idea. Kind of odd that its fine over 1k but below it the bounce comes back.