i had this happen on my bug a few times. the following is something ive found searching this topic, hope it helps:
High school physics: Latent Heat of Vaporization.
When stuff evaporates it takes heat out of all the stuff around it. When fuel leaves the carburetor venturis it is vaporizing (being ripped into little droplets) but it is also evaporating at the same time (so much more surface area to evaporate from with zillions of little tiny fuel droplets). Because it's evaporating it's drawing that heat needed to do so from everything around it- namely the carburetor and intake manifold.
This can drop the temps low enough that when the air is cool and moist it can actually cause water vapor in the air to condense on those (chilled) parts.
In really cool, wet weather (below about 45*F) that condensation can actually FREEZE on those parts after it condenses. When it does so inside the venturis/around the throttle plates in the throat of the carb it's called "carburetor icing" and it can choke the idle down to the point it can actuall stall the engine!
