YS engines will not run correctly without the check valve in the vent line. Although, on a YS engine it's not really a vent line but more of a pressure line. That line goes from the pressure outlet on the engine itself and then runs through the check valve into the vent of the fuel tank. Make sure you have the check valve set so that pressure goes through into the tank, and not backwards. The fuel pickup line from the tank then goes to the fuel intake on the engine. You MUST have it set this way or the engine will not run properly. YS engines are designed to run with a pressurized fuel system and that check valve is there in order for pressure to stay in the tank.

Ken