Yamaha RST90K Owner's Manual download pdf (Page 59)

8-5

Spark plugs are produced in several different thread

lengths. The thread length or reach is the distance from

the spark plug gasket seat to the end of the threaded

portion. If the reach is too long, overheating and engine

damage may result. If the reach is too short, spark plug

fouling and poor performance may result. Also, if the

reach is too short, carbon will form on the exposed

threads resulting in combustion chamber hot spots and

thread damage. Always use a spark plug with the speci-

fied reach.

Before installing any spark plug, measure the electrode

gap with a wire thickness gauge and adjust to specifica-

tion.

When installing the spark plug, always clean the gasket

surface. Wipe off any grime from the threads and tighten

the spark plug to the specified torque.

CAUTION

Make sure that the spark plug caps are securely

installed. Otherwise the spark plug caps could be

damaged due to engine vibration.

Spark plug reach

: 19.0 mm (0.75 in)

Spark plug gap

: 0.7–0.8 mm (0.028–0.031 in)

Spark plug tightening torque:

12.5 Nm (1.25 m·kgf, 9 ft·lb)

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Yamaha RST90K Owner's Manual Page 59