what makes it run differnet when cold?

[rbohm]

the more oxygen you can pack into the cylinders, the more power you will make. that is how nitrous works, it gives up the oxegen molecule so it can help burn the gasoline more efficiently. when you lower the air temp, you get a denser air mass, thus more oxegen, thus more power as you noted. it has been found that for every 10 degree drop in air temp, you gain 1% hp. and use the choke only when the engine is cold.