What year CAR are you working on?
What engine modifications?
What ignition (point, electronic, aftermarket) what brand?
What year Engine is the 318?
If rpm goes up due to a vacuum leak ( taking the cap off a port) it could indicate that the idle is too rich otherwise a vacuum leak would cause the mixture to lean out and the engine would run rough.
Keep in mind that a vacuum leak is the equivalent of opening the throttle blades a bit. But with out the benefit of the air going through the carb the correct way.
I assume you have tested all the vacuum ports under idle, part throttle and full throttle conditions.
That will tell you a lot about what each does.
If you have a flapper on your air cleaner to control heated air coming off the exhaust manifold you will need to consult the FSM as to what type of vacuum signal it needs.
Basically if a port is at full manifold vacuum at idle it's likely tied directly to the plenum in some way.
If it has 0 vacuume at idle but increases as the throttle is opened it is a ported vacuum but it might not be the correct one for vac advance depending where it comes from in the carb