The heat valve never fully stops the flow of heat to the underside of the intake, no matter how well it works.
I had my truck out on Sunday when it was 18 degrees, even with a heated intake, the two outer carbs were stalling out.
Fuel vaporizing pulls a ton of heat out of the surrounding metal and air which is great for a race car, but terrible for everyday driving and questionable fuel quality makes it even worse. The intake needs to be hot enough to replace some of the heat loss, but not so hot it vapor locks. This is where the flapper valves comes in. Its used to heat up the undersize as fast as it can then shut down and maintain a constant temperature.
My alcohal carburator would ice over so bad on damp nights I had to add heat under it just to make to the end of the quarter mile!
Joe
