Mercury - Fuel Valve 5-Inch Fuel System Component Marine Composite 8M0044687. Overview. There are a few small parts on a boat that you don't think about until the moment you really need them. A fuel valve is one of them. When you're heading out for a long morning run or easing back toward the dock with the sun dropping behind you, you just want to know that every piece of your fuel system is doing its job without question. That's why boaters appreciate a dependable Mercury Marine part like this