Some small Continentals O-200 and smaller have Stomberg carbs and they have no ICO. So this technique with those engines will result in surprises!
Mine had one of those, since replaced with a Marvel Shebler with a real mixture control, I still shut down with the mag switches, at least it "proves" they work. It is my a/c so no-one else goes near it except the farm owner & he has enough sense to not "poke the cat".