[Gaz64]

How much vacuum does the engine produce at idle?

What is the idle A/F ratio doing as the engine slows down?

With a big cam, you'll find any large electrical load (fans, headlights etc) will cause idle rpm drop. The better the idle emulsification and manifold distribution is, the less the drop will be.

The different loads the engine sees at idle is one of the reasons why GM ran an ILC on their '80s carbed engines.

Can also be fixed if the car has programmable ignition maps.