do you have mech. fuel pumps? if so then it has to fill the carbs up which takes awhile. if electric pumps maybe a weak spark to plugs to fire the cold mixture. in older motors if compression is a little lower it doesn't heat the mix as much when compressing and its harder to fire. If Q-jet carbs maybe loose throttle shafts causing vacuum leak enough to defeat the choke somewhat. just tossing some out need more info.