SCCA regulations are very restrictive in some ways. For instance, I'm limited to 40mm carbs to supply an engine capable of 180+ HP, and certain mods aren't allowed. However, getting them to breathe is only a matter of being creative and spending lots of time making tiny improvements.
These pics show some of the mods I thought up to increase the Weber 40IDF flow capability.
Stock secondary venturi on the left. My ultimate mod on the right.
Click to view attachmentI found some tiny airfoil tubing at the hobby shop. It took some work to fit it to my setup though.
Click to view attachmentKeeping the venturi in place is a bit more challenging now.
The long bolt holds it upright and against the fuel inlet on the other side.
I turned off the threads so the airfoil tubing will fit onto the bolt.
Click to view attachmentEvery surface/obstruction in the throat has been altered/filed/smoothed to increase flow.
The throttle plates have the edges re-worked to prevent turbulence.
Even the exposed pump jet tip has been file down.
Click to view attachment