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The only time to use the ported vacuum feature on the carb, would be if you had a functional EGR. (That would be the white circle/blue arrow in your picture).
It's best to run full manifold vacuum to the vacuum advance. The KEY is - you have to either run an adjustable vacuum advance (such as the Crane unit), or limit the travel of a stocker. Obviously, the adj. unit gives you more flexability!!
With the vacuum adv. off and plugged - set your initial timing to 10*-12*. Hook up the vac. advance and dial in the advance cannister to give you about 8*-10* more adv. See how that works for you....pay attention to pinging.
Next thing to work on is your total timing. You need to see how much mechanical advance you have and how soon it's coming in.
I'm more familiar with timing for Buicks - we shoot for a total of 32* or so, and have it all in by 2500rpm. Pretty sure Chevy engines like a little more total timing.
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Scott
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