I have this problem with my tachometer on my SR302 twin. The tact bounces and spins around at idle and when it is running. Have already replaced tact and checked the alternators and the regulators. Replaced the distributor,pick-up and coil. Checked all wiring.
Have gone as far as a limiting boat wiring, made-up short leads and hooked up directly to the coil. Still there is this problem. I have check the ground. Thank you

are you running tach adapters,if so make sure all
connection are good .

The HP500's had a grounding problem! Had to clean off the blue paint on the manifold. I'm sure you have checked the ground, but have you cleaned it?

If what Jeff said is not the problem lat me ask if you have MSD 6 boxes. If you don't disregard. If you do then you need to plug your tach signal into the side of the box. This is a common problem if you have an MSD unit.

Roby

No, I'm not running boxes and don't have adapters. Yes the ground is clean and have added a secondary internal ground. the only gage's that are effected are the tachs whether I'm running a single or both engine. I have also tried a isolated power source to run the tach.
Thanks for your help with this.

Are both Tachs jumping? If so do they jump in sequence?

If its just one Tach, I'd try swapping the wires and see if the problem follows the gage, or if its in the wiring.

No nothing changes even with new tachs and short wire loom , distributor, coil and pick up.