Re: Need help with 2 cylinders not firing all the time.
Tachometers don't usually just die, they don't usually work well forever either.
Most of the tachometers need a positive (purple wire) and a negative (black wire) feed off the ignition switch and a signal from the engine via the gray wire. If either wire is missing or the connection is broken the tachometer will not work.
When you first turn the key to the run position, the tachometer should receive the 12 volts and default to the last position it was in when the ignition key was turned to off, or zero, whichever the case may be. If that does not happen, recheck the positive and negative wires and clean the terminals.
BTW, when you are working with the tachometer, don't let any voltage at all get to the gray wire - that can instantly destroy your rectifier and set the system up for damage to the ignition system.
