You've got a classic case of over-carburetion. The velocity of the air going through the venturis is so slow as the carbs transition from idle to midrange operation that there's no venturi vacuum to pull fuel through the main jet. The carbs go lean momentarily, regardless of how much vacuum there is below the throttle plates at idle. Raising the float level(s) is one possibility, but then there's a serious risk of flooding. What you really need to do is find a set of carbs for a 216, and modify the manifold stud spacing to bolt the smaller carbs onto the manifold. You will still have far more venturi area than you'll ever be able to use, but the flat spot will go away.
Jerry
