2007-2009 Yamaha F15/F20 Outboard Service Manual, Page 289Get this manual
2) AUTOMATIC DECOMPRESSION DEVICE When the starter rope is pulled for cranking the engine, the weight arm pushes up the exhaust valve to delay the timing to closeTherefore, the compression stroke becomes shorter, causing decrease of the force to pull the ropeWhen the engine starts and it exceeds 550 rpm, the centrifugal force rotates the weight arm to defectAs result, the exhaust valve becomes no longer effected by the arm, and the engine continues to run normally
0.9 mm projection
#2 weight arm