erratic Tach readings.

[Ron La Savio]

What would cause the tachometer to give erratic readings when additional power is turned on like turn signals, air condition, etc, while driving. This happens after the car is warmed up and running.
Got a BBC in a 70 Chevelle SS.

[chevymaher]

Sounds like a bad ground is causing interference. I would take all grounds off sand them clean them up.

[pitts64]

Run a jumper cable between the battery negative lead and the block and take it for a ride..