Yamaha Outboard Motors Factory Service Manual F6 and F8, Page 450Get this manual
IGNITION SYSTEM CHECKING THE SPARK PLUGS
Check: Spark plugs Refer to "CHECKING THE SPARK PLUGS" on page 3-23
CHECKING THE IGNITION SPARK GAP WARNING
Do not touch any of the connections of the spark gap tester lead wireDo not let sparks leak out of the removed spark plug capKeep flammable gas or liquids away, since this test can produce spark Check: Ignition spark gap Above specification spark plugIgnition spark gap: 0.9 mm (0.04 in) Checking steps: (1)Remove the spark plugs from the engine(2)Connect spark plug cap to the spark gap tester(3)Set the spark gap length on the adjusting knobSpark gap tester: YM-3448790890-06754 (4)Crank the engine and observe the spark through the discharge window of the spark gap tester