1964 Johnson 18hp spark plug selection?

[nathangormley]

I have recently been running champion J6C spark plugs in this motor. I just read something that had champion J68 listed as the correct plug. Could somebody tell me what the difference would be between these two plugs and which one would be better? Thanks in advance.

[JB]

The original plug was Champioin J4J, Nathan, currently J4C. I believe it is still the best plug for that engine (FD-18), but many have switched to the J6C. Either will do the job, gapped at .030". The J6C is less vulnerable to fouling at low speeds and the J4C offers slightly better high speed operation.

I think the J68 you read about was a typo.

[nathangormley]

Correction instead of J68 I meant J8C. What would be the difference btw J6C and J8C?

[JB]

J8C is much hotter, too hot for that engine. J6C is about as hot as it will tolerate and J4C is sort of laid back.