Double pumpers only open the back barrels when you mechanically push the accelerator far enough to open them. That would not occur in normal driving.
The idea of the vacuum secondary is to give the motor the "right" cfm's as the engine needs them.
Both carbs can work well when set up properly. The "table talk" is that vacuum secondary is "better" for the street due to most cars having less gears or converter than a drag car or pro streeter, so to speak. My opinion, this idea is false. I've ran 750 double pumpers on very mild 318's that worked flawlessly. Was it more than the 318 needed? sure. Did it affect the running? nope.
