My guess is the flat spot in transition in Gordon's post above is in part (if not entirely) due to the mis-match of the relative heights between the bottom of the aux venturi and the waist in the main venturi (where the minimum air pressure exists and helps draw the fuel from the aux venturi.)
40mm Webers use 30mm through 38mm venturis successfully without modification of the progression circuit holes in the throttle body although they are certainly not optimized for the different displacement engines they are used on. I would therefore believe that venturi swaps are possible to be performed with correct geometry venturis. I've lately seen another forum post where someone is making custom venturis for Zeniths which would correct many of these issues...probably.
Jets, emulsion tubes and available tuner history are all against tweeking the Zeniths for specific applications.
Shimming the outside diameter of the Weber venturis sounds like a good fix for the tapered bore in the Zeniths but the geometry of the two venturis is still incorrect.
Paul Abbott
Performance Oriented