PURPOSE: To remove effectively carbon attached to a valve body by sinking the valve body in the concave portion of an engine after it has stopped for an exhaust system that moves the valve body provided near the exhaust port in order to change the height of the exhaust port and change exhaust timing.

CONSTITUTION: A valve body 44 is rotatably accommodated in a concave section 42 provided near and above the exhaust port 38 of an exhasut channel 34, and this valve body 44 is rotated to change the height of the opening of the exhaust port 38 by lowering the brim section 46 of the valve body 44 that is positioned opposite to the cylinder bore 18, so that the timing of exhaust is changed. In such an engine, the output shaft of a servo motor is connected to the rotating shaft of the valve body 44 by interlocking. And a load 60 that is the driving section of the servo motor is constituted to be energized through a transistor switch 78 for a specified period of time determined by a time constant circuit 70, when a main switch 62 is connected to an off contact point 76 while the engine is stopped. In this arrangement, the valve body 44 is sunk in the concave portion 42 to remove carbon attached on the valve body 44.