[Tom Goldman]

Had a similar problem several years ago.
Venting the bowls outside the scoop had a result similar to what you saw.
A friend who is involved with Pro Stock carbs suggested a couple of 1" holes in the scoop tray,adding or removing plugs untill the pressure sweet spot was found.
2 1" holes made a huge difference, but eventually raising the scoop to allow 4" over the carb was far more effective.