To explain it just a bit further, it is more of a requirement for Stromberg carburettors and the likes. With the way the Stromberg works, the needle and seat can freeze over from cold air. Makes sense, because the fuel that's evaporating off the needle will cool it down further, to the point of freezing. In cold weather, say under 5 degrees Celsius or maybe 10 degrees if it is very humid (?) the warm air flap is very important. At my car's first service (my father bought it new), the garage in Cape Town removed it saying it just give trouble and Cape Town is warm enough. I've now put it back, because here in Auckland, New Zealand the humidity is much higher and the Winters colder. Actually, Summer average is around 18 degrees Celsius (which is like normal office aircon temperature, don't know the Fahrenheit equivalent). My car now runs much better, especially in Winter. But the flap alse gets stuck. It's adjustable, and I think you can re-calibrate it using warm water. I've got more important things to do first, so that is on the waiting list...