Heck of a nice sketch Simon.
Salty, you can leave the inbord port open if you desire. The port on the end of the can advances the timing. Older cars only had this advance port on the vacuum cannisters connected to manifold vacuum. One of the first emission control strategies was to tie the vacuum cannister to a ported vacuum source located above the throttle plates. The vacuum is low at idle via a ported source, and this retarded the spark at idle raising combustion temps to more thouroughly burn the excess hydrocarbons. At speeds above idle, there is little to no difference between vacuum readings from a ported source and a manifold source and the vacuum advance will function normally at higher rpms. The next progression was to place a second port on the the opposite side of the diaphram in the vacuum can. This port is used to retard the timing and was routed to manifold vacuum. So at idle, the vacuum is stronger on the manifold port and the timing is retarded or not advanced. They also used the thermostat vacuum switch to keep the timing more retarded until the engine warmed up. Emissions are at their highest with a cold engine. Retarded timing provided hotter combustion temperatures to help burn fuel that normally may pass into the exhaust as unburned emissions during. The hotter temps also warmed up the engine faster. As time went on, they started adding more solenoids etc to the carburetor to allow it to cope with the retarded timing. Solenoids such as anti dieseling, fuel cut off, and air conditioner solenoids. They also started designed the distributors such that a greater portion of the total advance was controlled by the vacuum advance rather than the centrifugal advance.
