Plugs for a 1967 Johnson 40hp

[hodgie]

I have seen a lot of posts on here stating that I could either use J6C or J4C plugs. What is the difference between these plugs and which one would be better on this motor.

Thanks

 

[tmcalavy]

Re: Plugs for a 1967 Johnson 40hp

Depends on what you're doing with the boat...running WOT, cruising or trolling. The main difference between the J4C through J8C plugs is heat range...4 is coldest of the three, 8 is the hottest. Some people run a hotter plug when trolling (slow moving) to help keep the plugs from fouling. Whatever you choose, keep an extra set handy in the boat at all times. The stock plug for your outboard when new was the J4J, still available as the J4C, according to my chart for big twins between 1956-1970. Even the big Starflite 75 hp motors ran the J4J plug. Which reminds me...I need to check and probably replace plugs in both my big twins this spring.

 

[tashasdaddy]

Re: Plugs for a 1967 Johnson 40hp

http://www.boatsetup.com/Spark Plug Chart 2007 BRP.pdf recommends J4C, if you do a lot of idling, you can open the gap to .035 or .040, to cut down on low speed fouling of the plug.