When my car was running the tach acted extremly funny. It would bounce around like crazy and after a couple seconds at a steady RPM would settle down. When revving it jumps back and forth a good amount.

Anyone know what could be going on here?
Its a 1991 Z28 5 Speed

 

If it's accompainied by any missing/popping/hesitation it's often a sign of the mag pickup in the distributor going south.

 

The tachs in our cars are notorious for taking a crap. Mine is pegged at 7k, when I am really at 4k. Best you can do is pickup a new (used) one from a lower mileage 3rd gen from a junkyard or something.

 

To add to what Damon states -- check your ignition system. For example, an arcing plug wire can cause the tach to jump. Fouled spark plugs can also contribute to this problem. While the car is running, check your plug wires in the dark. If they are arcing you can easily see the sparks. Have you done a tune-up recently?

 

Its done this on two different motors. I pulled my perfectly running 305 in place of the 383. I am having to rebuild the 383 again...

 

Back from the dead...

Any other opinions?

Bad resistor in tach?
Something else wrong with tach?
Dizzy messed up somehow?
MSD box screwing with me?
Some higher being screwing with me!?!?!