Maybe you need cardboard in front of the radiator? I'm considering exhaust wrap on the arm of the intake manifold going to cylinders one and two because it happens to me too.
Pressure x Volume = n x R(constant) x Temperature - lower temp of air at the carb means that n (# of moles of atmosphere) has to go up just from the temp drop in the volume of the intake side. The air pressure in my tires has dropped without letting any out...
'Open Loop' in the carburetor world = running with the power valve open. It corrects for the above and would be a means to get more gasoline thru the system without having mechanical issues like overflowing float bowl, etc. My choke open, warm temp carb in summer has a really low idle at the red lights. In winter I can hear the engine above the radio, and idle is way up.
Your plugs suggest to me that fuel mix looks a bit lean - tan, not white is normal, but, I don't know that plug heat range? I use NGK BPR5ES, and Denso. My worst gas mileage was 8mpg. My best was 15mpg.
Coasting down the hill, other rig, the other day. Just idling down the mountain caused my aftermarket radiator to source my motor with serious cold coolant, and my block got so cold, that I had to choke it after I eventually had some landscape to ascend. The thermostat might be worth testing. Desmoged air intake, and removal of the original hood insulator is going to make for quite a bit more space to move air around there, and I have trouble keeping the aftermarket carb warm.
