Does the carb have kill bleeds in the primary venturi? If you pull the carb top turn it upside down and looking from the front on each side where the booster channel meets the housing there may be a pressed in plug with an orifice. I suspect that these are kill bleeds to delay the onset of primary booster flow but have never played with them to verify. If you have them blocking them may help the primary boosters to start flowing earlier.
Where do you start you're dyno runs?
