I think that when there is more moisture in the air that there is more oxygen
in the air too (remember that water is made up of 2 hydrogen & 1 Oxygen
molecule)- more moisture in air means more Oxygen getting into your carbs.
Its kind of like a Supercharger - introducing more air to the mixture makes
things work better or something. I'm no scientist, I just remember reading
something about this somewhere. I think that one of the Listers here is a
molecular scientist or something- maybe he can elaborate!
1969 MGB Roadster