One of my engines has a bounce in the tach at idle up to about 2500 RPM. I don't detect a miss and it doesn't seem to effect performance. Mechanic scanned it- no codes, compression OK, changed to plugs- still there. Any ideas?

Check your electrical connections

Thought about that- easier said than done of course but hopefully that's all it is-

gaffrig

Thanks- I'll start there...

I had exact same symptons a couple yrs ago. Removed and cleaned the wire connections on the back of the tach and also flipped the 4-6-8 selector switch several times. Solved my tach bounce.

I am having the same problem. Please let me know what you find out.
US1, could you please explain what the 4-6-8 selector switch is?

Short or bad ground somewhere

A switch on the back of the tach that corresponds with the # of cyls. your motor has.