I'm just beginning to add Zeniths to my offered services but one item I know is that the 27.5mm venturis used in the Zeniths will need to be upsized for your application. This is problematic to me especially from all the posts I've seen that say Weber venturis may be used in the Zeniths with a 1mm shim at the bottom to make up for the height difference. This leaves a little info out. Two items to add are of importance to me: 1) the Weber venturi is 46mm and its outside diameter is consistent while the Zenith (Solex) venturi is 50mm at the top and 46mm at the bottom and 2) the location of the bottom of the auxiliarry venturi should correspond to the location of the inner minimum diameter of the main venturi for maximum efficiency of fuel discharge when the main circuit is active - the location of the discharge for the Zenith is 13mm into its corresponding venturi while the Weber distance is 8mm. What this means to me is: 1) the fuel delivery for a Weber venturi in a Zenith body will not be as responsive/efficient as it should be and 2) the bore in the Zenith throttle body is tapered so there is no proper location of the Weber venturi once it is installed, especially since there is no grub screw to secure it like on the Weber.
I expect responses saying that the above is of no value and their Zeniths work great with Weber venturis but as I've just begun learning about Zeniths all I can report is what I see.
The emissions features of the Zeniths need to be disabled to make them better for performance applications. One advantage I can see is that the secondary venturis on the Zeniths are closer in height to those NLA tall secondaries that Webers used on race engines and by everyone wanting better throttle response.
Paul Abbott
Performance Oriented