Yamaha Marine Outboards F4A/F4 Factory Service Manual, Page 86Get this manual
INSP ADJ
GENERAL
Compression pressure measurement
Insufficient compression pressure will result in performance loss1Check:
9Valve clearance
Out of specification AdjustRefer to "VALVE CLEARANCE ADJUSTMENT" section2Start the engine and let it warm up for several minutes3Turn off the engine4Remove: 9Spark plug