It has to to do with the amount of vaporized fuel and combustion ability of the engine. A pump gas engine with lower compression will not burn the fuel as easily, so it needs more vaporized fuel to get it to light. Since it's cold, you have no heat in the induction to help, so you have to add even more fuel to get enough of it to vaporize. Add to that a larger cam with lower vacuum at idle and it's even worse. If it's a street car a carb with a choke is the way to go, if it's for race only you can just pump about 3 times before starting it. One other area depends on what 750 it is, the newer HP carbs have a relocated idle circuit that makes it harder to pull fuel through the idle circuit. The idle jet really needs to be at the bottom.