The smallest part is commonly reffered to as the venturi,and the smallest part is the part determines how much air is going to pass through the carb.The size of the jet determines how much fuel the engine gets and there is only one optimum mixture ratio,I believe it is 14 parts air to 1 part fuel.Each carb and engine combo will require a specific jet size for the amount of air it is getting,and that changes with air density as well.