...That Smaller Square port is the Exhaust 'Cross-Over'. It Runs through the Intake Manifold and connects to the Head on the Other side. It serves to HEAT the Manifold Plenum and aid in Atomization of the Fuel, Especially Needed in Cold Climates.
That port also Supplies the 'Exhaust Gas Recirculation' System.
In Very HOT Climates, This Passage could be Blocked, as in Your case; In an attempt to keep the Manifold Cooler. I'am sure others will chime-in soon.
Yes! As Doug pointed out, the port also heats-up the 'Choke Coil' (through a separate tube) Causing the (Automatic) Choke to Open Fully.
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