outside diameter of the exhaust camshaft 7 the pins 23 24 are movably fitted in the corresponding first pin hole 16 or the second pin hole 17 of the exhaust camshaft 7 each of these decompressor pins 23 24 is located near one of the cams 10 and an end of each of the pins 23 24 selectively protrudes from the pin holes 16 17 such that the pins can contact with the valve lifters 28 or tappets of the exhaust valves 3 in particular in the illustrated decompression mechanism 5 the first and second pins 23 24 open the exhaust valves 3 during at least a portion of the compression stroke to thereby reduce compression pressure preferably the end portions of the pins 23 24 that protrude from the pin holes 16 17 are positioned generally opposite to the crests 10 a of the corresponding cams 10 when viewed in the axial direction e g as shown in fig5 and fig6 thus as described above the exhaust valves 3 can be opened in the compression stroke by the pins 23 24 in this case the crest 10 a of the cam 10 for the exhaust valve of one cylinder is located at the bottom side in fig6 and hence the second decompressor pin 24 protrudes upward from the second pin hole 17 in fig6 while the other pin 24 protrudes downward from the first pin hole 16 in a conventional decompression system it is not believed possible to adopt a construction in which two decompressor pins move in two opposing directions such as that accomplished in the present decompression mechanism 5 the end surfaces of the pins 23 24 that contact the valve.