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Ported vacuum for vacuum advance (vacuum port placed above the throttle plate) originated to lower pollutants from engines. Back in "the day", before smog equipment, engines used full manifold vacuum. You merely adjust the idle screw to raise or lower the idle when changing vacuum source. Some engines run better with ported vacuum, many have better off-idle response with full manifold vacuum. Try them both and take your choice.
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Do you know why they call it "PMS"? Because "Mad Cow Disease" was taken. --Unknown, presumed deceased
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