Carb heat is not the same as what the heat riser is expected to do. When I flew (private pilot) carb heat was beat into your head as carb icing was a very real danger even on bright sunny days. Also carb heat is not intended to be a fuel economy device, but to do what it says... melt ice in the carb/venturi.
The runner length on an intake manifold can cause fuel delivery issues as the length of the runner is part of the equation that determines how much air can pass per unit of time and how much of a pressure drop. A good example of this is to put a pressure gauge on an air hose right after the regulator, it will read the same pressure that the regulator is set at. Add a 25ft hose and measure the pressure at the end and its not the same. Its less as the volume that the air has to fill is greater (length of the hose) and things like surface friction will affect how man CFM can pass. It matters even in runs of a foot or two.
Why do you think fenton double intakes give you more performance than stock? The amount of displacement of the motor has not changed, but the length of the intake to each cylinder is much smaller and more direct... improving performance.
Remember in the 40s-50s they still didn't know much about motors as there was no need to have very efficient clean cars. Today they study the heck out of every bit trying to get 1%, heck, 0.5% increase in performance.
