The integral pump will pull more fuel than the engine and carb actually can use so, unless you want to waste a lot of fuel that will be dumped into the bottom of the boat, you will need the return line. This is the big difference between a gas and nitro engine. In a nitro engine, the needle valve restricts the fuel flow into the carb to just what the engine can actually use, the fuel being pushed to the carb by exhaust pressure. The Zenoah integral pump actually overloads the carb with pressurized fuel. This pressurized fuel is then "metered" by the needle valve as it only allows through the amount of fuel the engine can actually use. Any excess fuel is then returned to the bag for reuse. This is the same principle used in a fuel injected engine used in your car or truck, but not in a carb equipped engine. That's a whole different can of worms
