Yamaha Outboard F15A F9.9C, FT9.9D F15 Service Manual, Page 148Get this manual
INSP ADJ
GENERAL
NOTE: Before installing the spark plug, clean the gasket surface and spark plug surfaceAlso, it is suggested to apply thin film of anti-seize compound to the spark plug thread to prevent thread seizure9 If torque wrench is not available, good estimate of the correct tightening torque is to finger tighten the spark plug and then tighten it another 14 to 12 of turn b9 Always use new spark plug gasket
Compression pressure measurement
Insufficient compression result in performance loss