What happens to an engine during a cold start in sub-zero temperatures?

I had a 71 Honda AN600 (2 cylinder car) air cooled, Keihin VV motorcycle carb. pull manual choke out and That thing would light off like gregcons Triumph, just a jab at the starter button (that was rigged) would start that little motor, it would just catch immediately. Must have been that Variable venturi carb that necked down to nothing and the choke. after about 15 seconds push the choke in to its half way detent, and then 15 seconds later push it all the way in and away you go. That was only in the coolest 40F mornings here in CA. Tjhose air cooled motors had great heaters too. Heat in about 30 seconds as it was pulled straight over the exhaust manifold, just pray you didnt have a manifold leak or you'd get a headache.