[Excaliber]

I prefer a vacuum source above the carb butterflies generally speaking. Which means NO vacuum at idle rpm, only vacuum when the throttle is slightly open. The other method is direct intake manifold vacuum, which is currently what I'm using on my ERA. You could make a case for either source depending on what you are trying to do.

Intake manifold vacuum gives you full advance at idle, which dramatically increases idle rpm. So you adjust the carb butterflies to slow the idle speed down. When you shut down the engine the idle speed without the benefit of advanced vacuum timing is ultra low, the carb butterflies are all but closed as well. This is one way to counter a 'run on' or 'dieseling' condition on engine shutdown.