These main venturi will fit the 2-barrel Weber 48IDA carburetors only. They will not fit the 3-barrel IDA Triple.
What size do I need?
The venturi size is the first step (after choosing the carburetor) in tuning a Weber carburetor to your application. It is tempting to use the largest size for the most airflow, but this can hurt performance. Too much airflow impairs low-end driveability. In fact, the car may not start or idle at all. And remember that the engine has a limit to how much air it can use. Feeding it more will not produce more power.
The book Weber Carburetors by Pat Braden (Part No. 5326) includes venturi selection charts based on engine capacity, use, and RPM.