Red58 is right on the money. One other possibility: is the vehicle blowing black smoke after it warms up?

Carburetor icing is a result of the "heat of vaporization" of changing gasoline from the liquid state to the gasous state. This is an endotheric change (it requires heat). The required heat is removed from the air thus cooling the air (and creating frost deposits). If the carburetor is too rich, it requires more heat to vaporize the fuel, and contributes to the icing problem.

Other than the solution suggested by red58, another possible solution is to install a air deflector in front of the carburetor to keep air from the fan being blown on the carb. Lots of imports in the early 1970's had an air cleaner with switch that allowed pulling either fresh air or heated air into the engine.

Jon.

Good carburetion is fuelish hot air
The most expensive carburetor is the wrong one you attempt to modify.
If you truly believe "one size fits all," try walking a mile in your spouse's shoes!
The Carburetor Shop