Mercury - Check Valve Assembly Fuel System Component Marine-Grade Construction 424811. Overview. Fuel flow needs to move one way and one way only - that's where the Mercury Marine 424811 Check Valve Assembly comes in. Built for Mercury 150-300HP EFI TwoStroke outboards, this small but essential part keeps your system pressurized and prevents fuel from flowing backward. It's designed and calibrated by Mercury to hold the exact blowoff pressure your EFI engine needs to run right. Made from marine-