of the rocker arm 16 is caused to move downwardly causing the valve 12 to open as the lobe 33 of the four stroke cam surface 32 moves beyond the follower roller 28 the rocker arm 16 is caused to move upwardly and the valve 12 is caused to close operation of the valve 12 by the lobes 35 of the two stroke cam surface 34 is the same as that described for the lobe 33 of the four stroke cam surface 32 the engine which has a combustion system suitable for both two stroke and four stroke operation can be changed from one operating mode to another by changing from the operation of the valve 12 from once per revolution of the cam shaft 36 or crank to twice per revolution of the cam shaft 36 this is accomplished by switching the engine valve 12 from following the four stroke cam surface 32 to following the two stroke cam surface 34 the first locking pin 42 operates to lock and engage the follower arm 24 for four stroke mode the second locking pin 44 operates to lock and engage the follower arm 26 for two stroke mode the third pin 43 ensures proper alignment of the first locking pin 42 to engage the follower arm 24 for the four stoke mode the fourth pin 45 ensures proper alignment of the second locking pin 44 to engage.