For some reason, starting in 1957, OMC started  recommending the slightly colder  J4J for their motors. Why, I don’t know. Maybe because two-stroke oils were improving and burning cleaner or  people were pushing their motors harder.

The actual use and condition of the motor will help determine which ones to use. Cold water use and tired, lower compression motors will probably like a hotter plug while motors pushed hard as in water skiing or steady full throttle operation will benefit more from a colder plug.

As noted, the old J6J’s cross over to modern J6C’s or NGK B6S while the J4J’s cross to J4C or NGK B8S.

OMC must not have told Champion about the change.  My ’59 Champion cat still lists J6J for the QD series.

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gschnelzer