Originally Posted by Mike D

" May be I don't understand the system - but with a choke on a carb you get a high idle by reducing or restricting the air flow don't you? "

The answer on that is, no. On a carbed engine the idle speed is increased using an eccentric cam on the linkage. This is both to warm up the engine quicker and to draw more air to compensate for the enriched mixture. Kind of a "robbing Peter to pay Paul" method. You can't use a controlled vacuum leak for engine speed because then you would have a low vacuum condition which causes problems due to the disruption of the venturi effect which is what carbed systems use to introduce fuel into the air stream.

On a fuel injected system you can and do use controlled vacuum leaks because the fuel is injected under pressure as an aerosol and does not require the vacuum to draw the mixture into the intake runners.