Abstract: A two stroke cycle engine has an engine block with a cylinder bore accommodating a piston. An exhaust port connects the cylinder bore to an outside of the engine block. An exhaust port control valve mechanism within the engine, according to this invention, includes a valve pivotally mounted in the exhaust port between the cylinder bore and the outside for rotation between an open position and a closed position. The valve defines an upper timing edge of the exhaust port. An insert fills a volume in the exhaust port between the valve and the outside, thereby improving flow efficiency of the exhaust port.