If you're looking at vacuum at the advance, do it during a road test. Put a T fitting in the distributor line and run a long piece of tubing into the cab where you can watch the vacuum gauge during actual driving conditions. I think you're going to be amazed at how seldom there's enough vacuum present at the advance unit to actually make it move. That's particularly true if you've got two carbs, since even a tiny bit of throttle opening kills almost all the vacuum.
For jet sizing, ignore the stamped numbers- - - - -They don't actually represent jet diameter. Invest in a set of number drills, 0 to 60, and use the shank of a drill bit, not the cutting end, to check the jet diameter. A bigger hole equals more gas- - - -richer mixture. That's also dependent on throttle opening, because as soon as you tip in a little throttle, the power valve opens up and bypasses the main jet for a "full power" mixture. Once again- - - -more carbs- - - -lower vacuum- - - -more time spent with the jets bypassed. Ideally, multi-carb systems really should have weaker power valve springs, but I don't know of anybody who makes a selection of different-tension springs for the Rochester B. GM definitely didn't!
Tuning a Rochester B is sort of like herding cats- - - -pretty frustrating!
Jerry
