I have the same problem with my 1971 350 SL.
But I have fix the problem.
I put a brass pice with 7 mm hole , in the rubber hose from the supplementary air valve to the idle speed air distributor.
The idle are now approximately 700 from the engine are cold to normal operating temperatur. Before the brass pice, the RPM goes up and down from 500 to 1500 before the engines has normal operating temperatur
It works for me.