1991 Yamaha Outboard Factory Service Manual 9.9 and 15 HP, Page 19Get this manual
ADJUSTMENT POWER HEAD Spark plug 1Check electrode condition and wear, insulator color and electrode gap2Clean the spark plug with spark plug cleaner if necessaryUse wire gauge to adjust the plug gap to specification3If the electrode becomes too worn, replace the spark plug4When installing the plug, always clean the gasket surface, wipe off any grime that might be present on the surface of the spark plug, and torque the spark plug properly
cRemove the cylinder head and install dial gauge to the cylinder
Standard spark plug: B7HSlO (NGK) Spark plug gap0.9 1.0 mm (0.035 0.040 in) Spark plug tightening torque20 Nm (2.0 kg, 14ftlb) Ignition timing adjustment Adjust the ignition timing as follows: 1Full advanced position