Manifold vacuum is entirely dependent upon the throttle blade covering up the venturi of the carb. Vacuum is only "seen" below the blade, Which is why you do not see vacuum at the ported source until the throttle blade moves past the port. As the blade moves towards full open, the possibility of creating a vacuum goes away, because there is no longer any restriction over the manifold. If you have measurable vacuum at WOT, you need more carburetor. Fuel is pulled into the airstream by the siphoning action of the air rushing past the booster venturi, not by any vacuum that we can measure. This siphoning is what draws the fuel out of the carb at every circuit that meters fuel into the engine. The pressure drop at the fuel port is the siphoning agent, and we can "see" this action by monitoring a vacuum gauge, as long as we are only trying to monitor the idle circuit. Tapping into the booster venturi to monitor the main feed circuit is not practical in the field, so we rely upon a visual indicator (seeing the fuel come out of the mains). As for the presence of manifold vacuum at the ported source under idle conditions, this means that you have the throttle blades too far open, and the idle circuit is not properly metering fuel, as the fuel out of the transfer slot is not controlled by the idle mixture screw. If you have this problem, close the primary throttles until the vacuum goes away on the port, and then adjust secondary throttle blade angle until correct idle speed is achieved. If this results in a secondary throttle exposing too much of the rear transfer slot, reset the blades to expose the proper amount of slot, and drill a hole on the downhill side of the secondary throttle blade, to replace the air you just took away by closing the throttle blades.
