Mercury - Quicksilver Diaphragm Assembly For 110-300HP DFI OptiMax Outboards 804533. Overview. Your fuel system's performance depends on precision parts, and the Mercury Marine 804533 Diaphragm Assembly keeps your OptiMax outboard delivering smooth, consistent power. Built for Mercury 110-300HP DFI OptiMax 2-Stroke engines, this OEM diaphragm is designed for use within the fuel rail, maintaining proper fuel pressure and flow. It's a small but vital part that helps your engine start cleanly, run