2005 Yamaha F90TR 90HP Outboards Factory Service Manual, Page 225Get this manual
Power unit Ignition system Check the ignition spark using the spark checker WARNING
Do not touch any of the connections of the spark checker leads Do not let sparks leak out of the removed spark plug caps Keep flammable gas or liquids away, since this test can produce sparksNo Check the ignition system output peak voltage
Good spark? Yes Replace the spark plug
Within specification? No Check the ignition coils resistance
Check the ignition system using the Yamaha Diagnostic System