this might go toward a tech question but at the same time I think it might be anti tech...

on the edelbrock website they have some short tech vids... the link Im posting is pretty much bassackwards of the way I set up a vacuum advance distributor... the way they are talking seams more like vacuum advance for a california emissions set up or generally an emissions setup...

the vid is titled "Carb Tuning - Erratic Idle and Pinging"

http://www.edelbrocktv.com/featured_landing.php?reset=true

that link is only a 30 second intro... i imagine the old 'ported vs. mainifold' vacuum debate is what you're referring to. if you have a non-emission car, i'd recommend manifold vacuum for your distributor advance cannister.

I found the clip and he says if you have some off idle pinging to use ported vacuum because manifold vacuum is causing too much idle advance... In the context presented it could be a fix if you have some off idle pinging.

On the other hand I would approach it much different since most performance cams tend to like a lot of low rpm advance. What one needs to remember is Edelbrock advice is going to be based on their matched systems that are the pride of their fleet, heads, cam, intake and carb...