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There is a lot of vacuum from the engine at idle speed, it's in the intake manifold BELOW the carb butterflies. That is the vacuum source for all things related to the 'car' not the motor. Things like power brakes and vacuum motors that operate the heater controls, or flip up the head lights on some cars. Generally speaking there is about 20 inches of vacuum available. ABOVE the carb butterflies, depending on how your carb is ported, there is no vacuum available until you open the throttle slightly.
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