...........One point that hasn't yet been covered is that all ported spark advance is not the same.......................There's specific, documented evidence of the two types of porting as early as 1975. I've seen differing spark port locations in the throttle bore as early as 1960..................... So I decided to try an experiment on my '73......................Driveability did not improve, it got worse.
My guess is that the driveability improvement with manifold vacuum spark advance is specific to the underhood configuration. Mine differs substantially from the setups on which manifold spark advance was used; I have no EGR, my base timing is about 8°, etc. Still, I can't help wondering why I remember seeing manifold vacuum at the spark port of various more-or-less stock carburetors on my '62 Lancer back when it was my dad's car. I'm pretty sure the vacuum porting particulars depended in the pre-emissions era on manual/automatic transmission, but back then I think it was manual-trans cars that got porting closer to manifold-vacuum-at-idle.