Cooler intake manifold

Upto a few weeks back I've been daily driving my '62 NY wagon (413ci) with blocked heatriser port in the intake. Temps were around 40-45F.
The car would start to run and idle rough and stall at slow speeds after a few minutes of driving... Why? The carb's idle-circuit was freezing up. Frost was visible on the outside front of the carb.
The issue was worse with higher air humidities.