These are a marvel of engineering.
George Herbert (Bert) Skinner designed the SU carburettor. It was first produced in 1908 by his younger brother Thomas Carlyle (Carl) Skinner. Unchanged in the way they operate, the Skinners Union or SU carburettors were fitted to production cars until 1993.
Chapter 5 gives a simple yet fascinating description of how they work. Something every owner should know.