Mercury - Fuel Line Coupler Composite Construction 8M0001721. Overview. There are a lot of small parts on a boat that you never think about until the day they fail. A fuel line coupler is one of them. When it's doing its job, you do not notice it at all - and honestly, that is exactly how it should be. Whether you are drifting over a calm flat at sunrise or pushing through a choppy inlet on the ride home, you want every connection in your fuel system to feel steady and secure. Mercury Marine par