On a 4 cycle cap up is preferable, as you might suspect the carb will act a little leaner off the bottom with the pump stacks down because of having to pick it's fuel up off the floor instead of being dropped into the airstream with the help of gravity, it's a small amount of difference, but it's there. On a 2 stroke application on gas they tend to run cleaner at idle with the pumps down, top end isn't much different once velocity gets built up. Jon