Quote:
Originally Posted by Barnsnake
Many "cammy" engines like a slightly rich idle to reduce off-idle bog at a speed where the big cam reduces the engine's efficiency. The acceleration pump simply isn't quick enough to cover it all.
With such an idle mixture, opening a small vacuum leak would speed the engine up.
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Correct.
The mixture needs be richer at low speed to counteract exhaust gas dilution within the intake, and charge loss out the exhaust during the overlap period.
While the engine may be happy idling at 900 with 6 in of vacuum, another sudden vacuum leak is almost like just cracking the throttle open a little more than the set idle speed.