Spark Plugs

Figure 5-23F. Types of Plug Base Deposits


Spark plugs should be replaced every 2500 miles. Use only the replacement spark plugs listed on page 5-1.

The 5R6 and 5R6A plugs have a resistor element to reduce radio interference originating in the motorcycle ignition system. Only resistor type plugs should be used with 1980 and later ignition systems.


Disconnect spark plug cables from plugs by pulling on molded connector caps. Connection is the simple snap-on type.

CAUTION — Do not pull on wires since this may damage internal conductor causing high resistance and reduction in firing voltage.

Use a deep socket wrench or special spark plug wrench to loosen plugs. Blow away all dirt from plug base with compressed air before removing plug.


Examine plugs as soon as they have been removed. The deposits on the plug base are an indication of the correctness of the plug heat range, efficiency and are a guide to the general condition of rings, valves, carburetor and ignition system.

Figure 5-23F. Types of Plug Base Deposits bustion. The deposits may be cleaned off at regular intervals if desired.

A. A wet black and shiny deposit on plug base, electrodes and ceramic insulator tip indicate an oil fouled plug. The condition may be caused by worn rings and piston, loose valves, weak battery, faulty ignition or too cold spark plug heat range.

B. A dry fluffy or sooty black deposit indicates a too rich carburetor air-fuel mixture or long periods of engine idling.

C. An overheated plug can be identified by a light brown, glassy deposit. This condition may be accompanied by cracks in the insulator or by erosion of the electrodes. This condition is caused by too lean an air-fuel mixture, a hot running engine, valves not seating, improper ignition timing or too hot a plug for the service. The glassy deposit on the spark plug is a conductor when hot and may cause high speed misfiring.

A plug with eroded electrodes, heavy deposits or a cracked insulator should be replaced.

D. A plug with a white, yellow or light tan to rusty brown powdery deposit indicates balanced com-


Use only a wire type gauge. Bend the outside or grounded electrode so only a slight drag on the gauge is felt when passing it between electrodes. Never make adjustments by bending the center electrode. Set gap on all plugs to 0.038 - 0.040 in.


Before installing spark plugs, check condition of threads in cylinder head and on plug. If necessary soften deposits with penetrating oil and clean out with a thread chaser.

Install spark plug finger tight and then torque to 18 to 22 ft.-lbs. If a torque wrench is not available, tighten finger tight and then using a spark plug wrench, tighten an additional 1/4 turn.

Check and adjust, if necessary, engine idle speed and mixture setting.

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