Harley-Davidson spark plugs (Figure 5-23) have been designed to give maximum life and efficient combustion of fuel. They are available in various "heat ranges," each for a particular service application. Plugs are labeled with numbers, the lowest number indicating the "hottest" plug. Designations such as 3-4 and 3-5 are special purpose plugs.

For normal service, the spark plug as recommended in motorcycle "Specifications," should be used on a particular model. However, for special service conditions, a "colder" or "hotter" plug may be desired. The number 3 plug could be used for slow speed or short run operation while the number 4 plug could be used for the higher speeds of highway travel or maximum throttle operation. It is not uncommon for best results to be obtained with plugs of different heat ranges in front and rear cylinders, with the front usually the colder.

The 5R6 plug has a resistor element to reduce radio interference originating in the motorcycle ignition system. The resistor element will not affect engine performance or fuel economy.

Inspect spark plugs for condition and electrode gap every 2000 miles.

Sparkplugs may be replaced or cleaned at the 2000 mile inspection depending upon the condition of the procelain and electrodes, however, a new plug will provide the best performance. The sprak plugs should be replaced with new ones at least every 5000 miles.

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