[Steve R]

It sounds like the carb is lean at that area of throttle opening.
You should purchase their tuning kit that has the various rods and jets. Take the car to a shop that has a dyno and an air/fuel ratio analyzer. have them tune the carb through the various throttle positions and loads.
That MAY get this carb to work. Personally, I don't think it is a very good design. The secondaries lack of squirters and their counterweighted "air valve" design doesn't do very well under aggressive throttling.
If there is someone out there who loves these carbs and knows how to get them to work as well as a Holley, I would love to hear from them.
Steve