The Gunson Colortune spark plug, another fine piece of work from our friends across the pond, the Brits. This plug is designed with a glass insulator to allow viewing of the combustion chamber in autos, cycles and other small engines. The purpose is to identify the color of the combustion and therefore determine the fuel/oxygen mixture strength and adjust as needed. Too rich, yellow/orange flame; too lean, light blue/white flame. Bunsen blue flame is just right.
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See Through Spark Plug!