weird ignition issue after installing tach

95teetee said

67Dart273- yep, I hooked the tach to the neg. side of the coil.(it still starts and runs fine as along as that ground wire is disconnected, .

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THIS SHOULD BE your huge clue

Either you have

the tach wired wrong

the tach is not compatible with that ignition (Tachs don't always hook to the coil neg, as in MSD)

the tach has an electronic fault/ problem