Got the picts of the carb, interesting about the IAB's missing in the front. Do you see any restrictions farther in the hole or in the intersecting passage? And can you measure the idle feed restrictions in the metering blocks? Did you get the carb new or used? An open bleed would need a lot of idle feed restriction and limit the time the transition circuit was active. You might get it to work, just might make tuning it a bit harder.