Mercury Mariner 4 and 5HP 4-Stroke Outboards Service Shop Manual 1999, Page 119Get this manual
CYLINDER HEAD
Valve Springs
1Check each spring under load on spring testerReplace any weak springs
Valve Spring Compressed Pressure (Installed)
Closed Height 0.965 in(24.5 mm) Open Height 0.709 in(18 mm)
171bs(74.5 N) 31 Ibs(139 N)
2Check free length limit of each springReplace if out of specification[55846