2000-2005 Yamaha F40B Outboard Service Manual, Page 454Get this manual
IGNITION SYSTEM
CHECKING THE SPARK PLUGS
Refer to "CHECKING THE SPARK PLUGS" on page 3-22
CHECKING THE IGNITION SPARK GAP
WARNING Do not touch any of the connections of the spark gap tester lead wiresDo not let sparks leak out of the removed spark plug capKeep flammable gas or liquids away, since this test can produce sparks
Check: Ignition spark gap Above specification spark plugspark gap 0.9 mm (0.04 in)