Do a WOT run for at least 400 feet . AT end don't let off throttle, keep it wide open and cut engine off. Coast to side and when engine dies let your foot off the full throttle position. Have your plug wrench with you and remove the plug. What color is the tip and porcelain as well as the base of threads. If white it's running lean, if black it's running rich. Tan to medium brown is good and will not be the cause of the problem. Let us know what you find.
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