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Many people have issues with the kawasaki FJ400 engine fouling out spark plugs, after working on a few FJ engines and 600 series Mule's with this issue, i can say the fix for this issue is using a slightly hotter spark plug... the factory recommends a BPR6ES NGK plug.... the Number in NGK plugs represents its heat range and somewhat odd, the lower the number the hotter the plug, at least as far as NGK is concerned. If you have a Mule fouling spark plugs try installing a NGK BPR4ES spark plug. FROM NGK A hotter heat range spark plug has an insulator design with a longer heat flow path to the metal shell of the plug. As a result, more heat stays in the ceramic firing end and less is dissipated to the engine. A colder heat range spark plug has an insulator design with a shorter heat flow path to the metal shell of the plug. As a result, less heat stays in the ceramic firing end and more is dissipated to the engine. For a spark plug to function properly, it must have a tip temperature high enough to burn off carbon deposits (self-cleaning) and avoid fouling, while remaining low enough to avoid overheating the ceramic firing end and pre-ignition.