Baby it's cold outside.

Your carb is icing up, it dose not have to be freezing out for this to happen, it simply happens when the volume of air rushing through the venturi of the carb ( like an hour glass ) rapidly cools the air and ice crystals form inside the carb, sometimes it actually looks like a little snowball in there, if you lift the lid from your air cleaner while running you may see it, it was always a common problem on many cars including Chrysler products through the eighties even, you simply need to make sure the warm air from the exhaust manifold reaches the carb through the bottom of the air cleaner snorkel, if all is set up right once the choke is off the engine should start to be warming up and as the manifold gets hot there may be a thermostatically operated door in the bottom of your air cleaner snorkel that will open and close to allow warm air in however on a 70 I am not sure if that was out yet may be too early if so it is not difficult to fashion a stove pipe ( cardboard style heat proof tube you could purchase at Napa or ) to go from the manifold to air cleaner bottom, that should keep the carb from icing, google it you will see what I mean.