That flat spot makes me think it's lean at that point. I'd try a quick test of putting my hand,or hands over the carb to restrict air intake to richen things up just to see what the result was.
It' hard to comment on that Offy manifold but I do know sometimes it's possible to get great volume but a lose of needed velocity in the air/fuel delivery. Which is to say, the fuel mixture is there but at some point is not moving to the cylinders as fast as it is needed.
It's something I ran into when I modified a motor.
Dan
