[Excaliber]

Myself, I use the vacuum advance on my distributor. The vacuum advance increases my idle rpm WITHOUT having to open the carb butterflies to increase the idle rpm. When the motor is turned off and the electric timing (distributor) is no longer in play the carb butterflies are all ready closed off enough to prevent more fuel/air from being drawn in. Similiar to the solenoid approach, closing off the carb butterflies in some manner. In the early days of pollution control which required a higher idle speed the solenoid method was a common tactic used by many manufactuers specifically to address the run on issue.