Sorry to revive an old thread, but it occurs to me that this port is before the throttle butterfly valve. My vac advance isn't doing anything at idle, and there is no vacuum signal from the port on the carb body.

I would think to get a proper vacuum signal you need to have the port after the butterfly. can someone shed some light on this?

If there's no vacuum advance at idle, it's operating exactly as designed. It's known as a "ported" vacuum signal and it's supposed to peak right off idle at light cruise pedal position.

Here's some discussion and theory about such things:

http://www.classiczcars.com/forums/electrical-s30/44790-ignition-timing-theory-port-source-vs-manifold-source.html