Make sure the warm air pipe from the exhaust manifold shroud is present & hooked up, and that the shroud is mostly intact. Then check that the flap in the front of the airbox actually opens, and stays open 99% of the time. If it doesn't, check the 2 vacuum lines on the side of the airbox are hooked up, and that vacuum is getting to the front pipe.
One thing you can do is hold the flap open with a self-tapping screw, it should be open pretty much all the time even in hot weather!
Next step is to check that the little heater on the front left side of the carb is plugged in & getting 12v, and that the earth strap from the top of the carb to the rocker cover is hooked up.
Edit: and here's the science bit
The pressure decrease in the venturi of the carburetor causes cooling. Cooling causes condensation, as cool air can hold less water vapour than warm air.
Vaporization of the fuel requires heat, which is absorbed from the air, causing the air to cool further, and causing further condensation. The combined cooling may result in as much as a 30 C temperature drop in the carburetor.
Ice can therefore form when the outside air temperature is as high as 30 C, but the most likely temperatures for carb ice are between -5 C and 15 C. High humidity conditions will make the chances of carb icing even higher. The ice usually accumulates around the discharge nozzle, the venturi and the throttle valve, restricting fuel flow.
