Sounds like your vacuum advance line is connected to full manifold vacuum. If it were connected to a ported vacuum it wouldn't make much difference at idle whether you disconnected the line or not, at least in my experience. However, even with ported vacuum, when messing with timing I still disconnect the vacuum advance just to be sure. A ported vacuum source will be above the throttle blades therefore at idle will have barely any vacuum. Many older carbs have full manifold vacuum. Seems more carbs were built with ported vacuum as a result of the Clean Air Act in the 60's.
With full manifold vacuum, if you didn't disconnect line, your engine would probably crap out before you got anywhere close to what the book says your timing should be. So, you set your timing with vacuum line off and plugged, then when it's set, you re-connect and yes, it will advance because you have full manifold vacuum. The timing will also increase even more (to a point) as you increase throttle. Does this make sense? Sometimes I get longwinded whereas other people can make the same point/statement in 3 sentences.
I dealt with this same issue earlier this year, er, last year when I changed my intake and was experimenting with different carbs.
-David
Kustoms-and-Sleds
