Mercury - Adapter Harness Racing 350 SCI to 400R Fuel/Paddle/Vessel Interface 84-8M0137199. Overview. Repowering from a 350 SCI to a 400R can leave you with harnesses that don't quite match, especially around the fuel-level, paddle-wheel, and vessel-sensor connections. The Mercury 84-8M0137199 Adapter Harness makes that transition clean and straightforward. It bridges the older 350 SCI harness layout to the newer 400R interface so the sensors you already have can continue talking to the engine w