TinyJaime,
Here is how I plumbed the "ported vacuum" to the distributor on dual Carter-Webers. There are ported and no-ported vacuum ports on the side of the Carter-Weber carburetor.
Simple definitions:
ported vacuum - above the throttle plate/flapper in the carburetor - little or no vacuum at idle - this is where the vacuum advance should be connected
nonported vacuum - below the throttle plate flapper in the carburetor - vacuum at all times when the engine is running
For the heck of it, I ran a tee-ed hose w/fitting from both carbs to the distributor vacuum advance unit (I thought it would look nice).
You can also see the port on the side of the intake manifold that runs to the wiper motor. This is non-ported vacuum (it has vacuum all the time) - this is not appropriate for the vacuum advance.
The gas going into the combustion chamber and flowing as exhaust gases out the exhaust creates the vacuum (another simplistic explanation) - vacuum sucks (sucks gas/etc towards the combustion chambers in the block). That is why gas does not go into the wiper/brake-booster - the vacuum is sucking from those devices. When you get a back fire on the carburetor side of the valves (as apposed to an exhaust-side backfire), that back-fire will blow towards the wiper motor and towards the vacuum brake booster (the vacuum is gone).
