Maybe this will help...

http://www.geocities.com/tetraethyllead/vac1.JPG
http://www.geocities.com/tetraethyllead/vac2.JPG
http://www.geocities.com/tetraethyllead/vac3.JPG
http://www.geocities.com/tetraethyllead/vac4.JPG

Book is Automotive Engines, 4th ed, Crouse, 1971.

It describes how the ported vacume works with the advance. Doesn't really
say why it's better, other than there is no advance at idle by doing it
this way.

   I may get flamed, but if my memory is failing on this, we'll both find
out.  IIRC, the ported vacuum increases with acceleration due to more air
passing thru the carb from the atmosphere.  I KNOW that when you accelerate,
manifold vacuum decreases (Proof comes from days of vacuum-driven
wipers--they slowed/stopped wiping when you accelerated to pass a car, just
when you needed them the most!).  If this logic is correct, ported vac.
increases timing when you accelerate.  The reason your Stude. likes the
manifold vacuum at idle is that its timing is advanced by the constant vac.
from the manifold.  It would have zero vacuum from the (closed) port!
   Should your car enjoy the timing so high that it kicks on the starter,
yet pops back when you reach higher(highest) rpm's, find some good info on
shortening the advance curve.  That will allow you max. advance to 'get out
of the hole' PLUS reduce the total advance on top end to eliminate popping
back--gives the best of both worlds & makes it feel like a new cam!  s

NAte,
I don't know if I can shed some light on this for you or not
but, I am going to take a stab at it.  

One of the big problems with performance cams is their lack
of vacuum at idle.  This results in a poor signal to the
carb ports to control fuel flow.  In order to get enough air
to idle, the throttle valves have to be open too far and end
up exposing the transfer ports.  As you have noted, this
also provides vacuum to the advance port as well as
supplying the fuel/air mix.  As the fuel mix flows the
vacuum rises advancing the timing which will result in a
lean condition producing a reduced vacuum which in turn
reduces the fuel mix flow from the transfer intil the vacuum
drops and the fuel mix richens again.  It is a seesaw
process that results in poor idle.  The idle port is still
the best place to get advance vacuum because that is related
to engine load which determines how much advance it really
needs.  The problem is to get the throttle plates to cover
the transfer slots at idle and cut off the vacuum to the
advance.  I have neve done this on anything other than a
Holley as most of the Carter AVS type carbs are pretty well
dialed in.  On the Holley's, you can drill a 1/8" hole in
each of the primary throttle plates about half way between
the shaft and the edge of the plate.  This allows it to draw
air away from the transfer port while the port is closed.
The vacuum is then not available to the advance at idle
making the vacuum signal to he idle ports more consistant
and smoother.

Again, I do not know if this procedure works as well on the
AVS as it does the Holley.  If you do this and it does not
work, you will need to solder the holes or rivit them.  I do
not know Edelbrock's posittion on this but, Holley tech has
no problem with it in performance apps.

Also, if it has an idle adjustment on the secondaries, you
may want to be sure they are adjusted correctly.  If you
have a secondary vacuum adjustment, you can sometimes help
by opening them a hair more such that some idle mix is
available from there which would give you more control over
the primaries and the vacuum signal.

Good
Lugnut

The good things about manifold vacuum:

-More advance at idle = cooler exhaust, less fuel burn at idle
-Less advance during cranking- easier cranking

The bad things about manifold vacuum:

- Lots of engines will bog badly (just flat fall on their face) when
coming off-idle, because the vacuum advance decreases as the throttle
opens, especially when its opened quickly

- More nitrogen oxide emissions at idle

- sometimes you get a mild misfire at idle

Bottom line- if a modified engine seems happier all around with manifold
vacuum, by all means use it.

First of all the gasoline available today is not the same as what that
engine and compression is going to like much. The first thing you need
to do is verify the vacuum advance diaphragm is good and not leaking. If
there is a leak in the diaphragm or vacuum lines none of the following
applies. When you say the RPM increases when you connect to manifold
vacuum that could be because the timing gets advanced at idle or it
could be because the engine is sucking air thru the vacuum line.

    As for your direct question. The only way you are going to know for
sure what the difference between port and manifold vacuum is is  to put
a vacuum gauge on each  and compare. I suspect if you did that you would
find that there is not much difference except at idle. Under load and
open throttle the ported  vacuum may be just a slight bit lower. If the
engine knocks (pings) then hooking up to ported vacuum should help
prevent that. If it doesn't knock then manifold vacuum shouldn't hurt
anything and may even help. This assumes you have a muffler that works
well enough to hear a ping if there is one. Ideally you want the timing
set so that it is just bit retarded from the point where it pings under
moderate and heavy load.  

    Hooking the vacuum advance up to manifold vacuum could cause hard
starting and rough idle when the engine is hot. That might be a problem
in summer but not winter. That might be a problem if there is a heater
for the air intake, but probably won't be a problem if the engine is
sucking cold air. Heat at the base of the carb and intake manifold may
be a factor also. Mostly they started hooking the vacuum advance up to
ported vacuum instead of manifold at the same time they started putting
heat stoves on the intake and the octane of gasoline started to drop.

-jim  
   
   

    If it runs smoother at idle hooked up to manifold and the distributor
vac advance is working then that suggests your base line timing may be a
little late.

    In general it should probably be hooked up to ported vacuum