Ran across your question while looking for info on installing bushings in my Rochester throttle body. I just rebuilt my Rochester and the instructions said to NOT use a gasket if an insulating spacer was used, so I looked at the bottom of my carb and, sure enough, there was a hole there that the gasket would block. The hole lined up with a cutout in the insulator. Near as I can figure, the hole is connected to the power piston. If the gasket blocks the hole, the power piston does not receive manifold vacuum and the carb thinks you are running full throttle all of the time and runs rich. The manual said that the power piston activates when manifold vacuum is less than 5" of mercury, I think.
