What would cause the vacuum advance to be unresponsive to 20in using a Mighty Vac. It moves probably about 0.5 mm and then moves back to position after I release the vacuum pressure ( I'm just guessing on the half millimeter, it just ever so slightly moves ). I did install a Patronix unit but I don't see how that would effect the move-ability. Don't know what else to check please help.

Sounds like the plate is stuck. Remove the vacuum unit, remove the breaker plate, clean and lube the plate, put it all back together the way it was, and try it.

The Cap'n

Okay
I'll try it now

I was hooking the Vac up wrong (wrong nipple on the diaphragm). So I guess my backfire at a steady rpm without load isn't caused by the vacuum advance. Under load the engine doesn't backfire.