AFAIK that's synchronous (both barrels open together), either 360 or 390 CFM @ 3.0 Hg" (if rated as a 2-bbl.) depending on the venturi size (27mm or 30mm).
I'll have to measure my venturi's one day to get the exact size but I did read the carb is synchronous. Seems to be mixed feelings about running/fine tuning the Weber carbs.
Come to think of it, my internet research shows that's how people tend to feel about all carbs while everyone's favorites are different.
Many years ago I had a 327sbc bored 20 over w/a Chet Herbert cam, camel back heads, headers bla bla bla....ran incredible on a rebuilt Quadrajet, only ok when I swapped it for a Carter. Although I had a Carter tuning kit and was able to swap the needles in the parking lot at will and that was pretty cool, never did run as strong as the Quadrajet.
