Abstract: The angular timing relationship of the cams opening and closing the inlet and exhaust valves of an internal combustion engine can be varied while the engine is running to improve combustion efficiency. The angular relationship is varied by shifting of idler rollers 78, 80 acting on the camshaft driving timing chain adjusted by providing two cams 108, 110 for the same valve intake 104 so that cam 108 controls opening and cam 110 controls closing. Exhaust valve 105 has its opening controlled by cam 115 and its closing controlled by cam 113. An idler 122 between the two camshafts controlling cams 110 and 115 acts on the timing chain. The timing is also varied by moving the pivot point 104 of the rocker arm 62 to obtain later closing of the inlet and earlier opening of the exhaust valve 46 at high speed. Valve timing and duration is also controlled by adjusting the length of the push rod 108 and its point of contact with rocker arm 62.