You're right, Paul. All the ports on the carbs pull a vacuum except the ones that Tom left open. Those are the ones closest to the air box to ventillate the float chamber. If that's plugged up, there will be problems.
If those ports are connected to the Charcoal Can and thereby the Anti Run-on valve, when the key is turned off, the ARV will transfer the intake manifold vacuum to those ports to shut off the fuel to the jet. Or at least that's the theory.
I found it surprising that so many people, including myself,don't need the anti run-on valve.