Cold air = rich, I think, because high altitude = lean.
You do not mention the temperature, but the most likely problem is condensation or icing in the carburetor caused by the additional cooling effect of gas evaporation. Standing still allows engine heat to warm the carb.
If you can rig it, some cars get around this problem by directing warm air from the exhaust manifold to the air intake. I imagine that the cylinder fins would also be warm enough. You may even be able to fashion an air funnel around the carb to warm up the outside of the carb with engine heat.