You only want to use ported vacuum from one of the carbs. I'm not familiar with the Zenith carb and it's entirely possible they don't have a ported vacuum port.
With that said, a ported vacuum port will be located just below the throttle plate.
1st locate where the throttle plate cross shaft is where it's ends are on the carb housing then look just below all the way around the carbs housing for the ported vacuum port. You might find one thats plugged
Good luck
