carb ported and manifold vacuum question

You need waaayyy more than 5* initial....sounds like maybe it was a Chevy shop that did it???... as thats the way Chev's are setup...using manifold vacuum on advance.

You have to limit the mech. advance in the dizzy to run a bunch of initial, or you'll ping like crazy....but you found that out already. Somewhere 34-38* total advance (init. + mech.) , then add about 10-15* vacuum on top of that, should be ballpark, all depending on what your engine likes.