The leaf valves keep too much back pressure from re-entering the carb on two-stroke engines. During combustion and the start on exhaust sub-cycles, they are closed because of the pressure as the piston moves back. If one is loose, it will not open all the way causing not enough fuel and air entering the engine. If part of one is broken or bent in, it won't close and the carb won't draw air well enough. My OMC service manual calls for a 1/4 inch gap on the valves, but my motor is a smaller, older Evinrude. There are two screws that hold them on, you should tighten those up if valves look fine. I tighten them up every time I take the carb off.
