Mercury Mariner 30/40 4-Stroke Outboard Service Manual 1998, Page 205Get this manual
CYLINDER HEAD
CleaningInspectionRepair
Camshaft
Manual start models are equipped with cam shaft featuring compression release mechanismThe compression relief mechanism releases percentage of the cylinder compression during engine cranking which lowers the starter rope pull forceAt cranking speeds (RPM) the cam shaft decompression levers rest against the side of the exhaust cam lobes, protruding out from the heal of the lobeThis protrusion contacts the exhaust valve rocker arms during the compression stroke, slightly opening the exhaust valveWith the increase of centrifugal force at engine running speeds, the decompression levers swing out of contact with the exhaust valve rocker arms, allowing the exhaust valves to operate normally (fully closed) during the compression stroke