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Spark Plugs, J4C or J6C?

T

toddjo

New member
Folks, I'm aware that the recommended plugs for my 1966 3hp Lightwin are J4C's. However since my complete tune-up, which was incredibly successful, I'm trying to fine tune things even more. If I slow troll approx. 70% of the time WOT the rest. Should I run a hotter plug like the J6C? And, and if so, is it safe for my engine? Thanks. I could have never done this without your help.
 
The recommended plug is as you state, the J4C, however the alternative plug is indeed the J6C, also recommended by OMC and perfectly safe to use. The gap is .030 . Usually it does provide somewhat improved performance.
 
I would use the J6C. It is the newly recommended plug for your motor and will help reduce fouling when trolling. You may do this already, but when I troll for a while I make sure I open up the throttle wide open for a minute or so before I take it back to the dock, just to try to burn off that carbon that might have built up. Also regular seafoam treatments never hurt a motor that is used for trolling.
 
The rule used to be -------Motor with thermostat used J4J-------And without thermostat used a J6J ---------The motor with a thermostat will run warmer so a cooler running plug can be used.---------Heat range of plugs has to do with how the plug gets rid of heat.
 
My guess is that OMC designed these motors assuming use at the higher RPMs. As racerone indicated, they would be concerned about the removal of heat and probably felt the J4C was the better choice. Then they sold the motors and noticed that a lot of people use the smaller ones for trolling and were starting to complain about plug fouling and eventually piston rings sticking, etc. They then ran their tests with a hotter plug (J6C) and observed that the motor removes enough heat at the higher RPMs and they work better for trolling and came out with their later recommendation for the use of J6Cs.

This is all conjecture on my part but that is what I suspect happened with your motor and some of the Hp motors in the 7.5Hp and below range of that era.
 
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