Reply To: 1955 25hp Johnson RDE-17 spark plugs

[David Stanton]

I don’t think changing spark plug heat range will make much, if any difference for smoke. The heat range is all about keeping the spark plug firing tip and insulator clean, keeping it warm enough to burn off deposits and cool enough to not self-destruct the plug, cracking the firing tip insulator, and causing pre-ignition. It has to do with how well the insulator for the firing tip is heat-sinked to the spark plug threads to conduct heat away from it. I would only use the hotter J8C if you mainly idle, like for trolling or lazily idling a pontoon around the lake. If you primarily run at higher speeds, the cooler J6C will be better.

If spark plug tip becomes too hot, glowing red, it can cause pre-ignition which is like hitting the piston with a hammer at the wrong time.
Dave