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what would those undesireable symptoms be??
I've never really had an issue, but the possibility exists. You could end up with an idle thats a little bit tough to idle down. The most likely problem is an idle that drops too far once you get it into gear. It could idle at 1300 in park/neutral but drops to 700 in gear because the vacuum drop once in gear gets exacerbated when the timing retards with it, further reducing vacuum. This is most likely to happen in a low vacuum, low initial advance situation like yours. I suppose you could end up with a strange off idle transition too.
Either way it wont hurt to try. Some people freak out at the idea of using manifold vacuum. I remember having a long, argumentative type thread on here a few years back about this. I've since noticed that people keep coming on saying that FBO recurved their distributors for manifold vacuum and set it up for manifold vacuum. Although I never set up a curve any different, I set the idle where the engine likes it and total where it likes it. Then I add the manifold vacuum to further increase idle quality.
The only difference is that the ported vacuum port SHOULDN'T pull vacuum at idle. It will if it is exposed from the throttle blades being open too far, i.e. the carb is off the idle circuit. Ported vacuum is actually an emissions device, and cars used to run manifold vacuum for years.
Now I need to pin those needles, got to feel that heat
Hear my motor screamin while I'm tearin up the street
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