I think that it is a vent port, possibly to the charcoal canister. I can see the bowl vent to the right, and ported vacuum signals are lower, near the throttle plates. Sometimes, there is a "venturi vacuum" port near the center of the carb, used to control an egr vacuum amplifier, but that is not what that port is, because it is above the venturi.
/wwwthreads_images/icons/tongue.gif Nice picture. Nice mahogany table, and a very clean carb. That port is the one that the carb uses to "sniff" out the vapors from the charcoal cannister. /wwwthreads_images/icons/wink.gif Not the signal port, the big hose port marked carb bowl.
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