My bet it's a combination of items. As a test disconnect the linkage from the outer carbs. Run just on the center carb. My bet is it'll wake up the bottom big time. The solid carb linkage is not the ticket for low end street use If this does indead fix it, consider going back to the factory vacuum secondary linkage. Even the factory stuff can be adjusted to open to quick if you run the lightest springs in the vacuum pods.
Doug