Mercury - Fuel Line Coupler Fuel System Component Cast Aluminum 30635T3. Overview. When it comes to older Mercury and Mariner outboards, a good fuel connection means dependable performance. The Mercury Marine 30635T3 Fuel Line Coupler is built for TwoStroke engines from 1971 and earlier, giving you a reliable, tight connection between your portable fuel tank hose and the motor. Its twist-lock design makes attachment quick and secure, while the cast aluminum construction holds up against salt, fu