Years ago installed a dash tach in the place of the clock in my ‘72 Charger. Ever since then when the right turn signal is turned on the tach bounces from zero to the idle rpm. The left signal does not do it. Seems it would be a ground issue. I have the cluster out working on other mods so wanting to address the issue now. Any ideas what to do or look for?

Here’s a picture of the back of the tach. Screwdriver painting to the hole for the right turn signal socket.

Does the other socket on the bottom do anything to the tach when it's actuated? Looks like the tach grounds through the two sockets? Maybe just run a separate ground to the body of the tach?

The bottom socket is for the instrument light. No problem. The turn indicator ground strap screw was not tight. I am hoping after tightening it more, that will cure it. I think the extra ground is a good idea.

The tachs housing is somehow coming in contact to the lamp socket ground. Insulate the tach body and isolate the tach's ground. A lamp is just a resistor that glows when energized. It does not block the + potential when energized so your tach ground or housing at off position is fine. The minute you actuate the turn signal you are introducing a high resistance + potential to whatever is touching that lamp socket.