Timing Marks on the 273 [66 Barracuda]

The ported or "timed spark vacuum port" as Holley calls it has zero vacuum at idle. That's the whole meaning of "ported vacuum" versus "manifold vacuum". The only reason it is advised to disconnect and plug the vacuum advance hose is " just in case" the idle speed is high enough to actuate the vacuum advance can. Once timing is adjusted correctly and idle speed is correct, the vacuum advance will not make any difference whatsoever at idle. If it does, you do not have it connected to ported vacuum. Ported vacuum is generally any vacuum inlet above the throttle blades, but there are exceptions, that's why it's best to check with a vacuum gauge.

Now, if we are discussing connecting the vacuum advance to manifold vacuum, then completely different rules apply.