Which Spark Plug for 5 HP?

[Demerest]

The previous owner has a J8C in this motor. Isn’t the RJ19LM the correct plug? Would the conversion to a Magnetron ignition have anything to do with a different plug?

 

[Ginz61]

The C8J and RJ19LM are the same heat range of hot. The RJ19LM is resistor plug for RF noise suppression. In older engines that have borderline compression the C8J will give better performance as it will more spark energy at the tip. Both are 14mm with 3/8" reach.