'53 super fastwin....J4C or J6C?

[mark in new jersey]

Simple spark plug question...or so I thought.

15HP super fastwin 15012...my manual says to use an (obsolete) J4J. Well, that crosses to the modern J4C...which is fine. J4C is readily available.....but a lot of people seem to be running the slightly hotter J6C. Why? Is there a reason that I'm unaware of?

I've long ago learned not to ask what's "right" or "best" on an internet forum!....but still: if all of the old manuals call for an (obsolete) J4J, why are so many folks using J6C instead of the J4C?

Is there a definitive answer?

Simple question #2: I read a lot that says "Only use Champion" or "Only Champion is approved by OMC", etc. But my 'rude service manual lists Champion, Autolite & AC numbers. If I want to run J6C, and my local Autolite dealer hands me a "303" [equivalent], is that a big deal? I really can't see how it would be, but I suspect some of you know better than I...

So have at it: what am I missing?

 

[HighTrim]

Re: '53 super fastwin....J4C or J6C?

Is that a factory OEM manual? All my charts say J6J(J6C) for your motor, which is what I run in mine, gapped at 030, but I troll alot with mine. Just run better because of the heat range. I ahve run the J4Cs but used 040 as a gap.

I do know that the OMCs were designed to run with the Champs, so that is what I use. Others, like NGK, are similar heat range, but the length is off (or has been) and have been known to have the gaps slammed close from the piston.

As you know, in a forum you will get every opinion under the sun, just use what you feel is best.

 

[JB]

Re: '53 super fastwin....J4C or J6C?

You can run anything you please, Mark. I would run J4C Champions gapped at .030".

J4C because that is the plug OMC specified for "normal" use. If you do a lot of trolling or idling J6Cs will be less likely to foul but not as good at higher speeds. If you still have a fouling problem at idle speeds try a .040" gap with either plug.

Champion because that is the brand the engine was designed to run. So-called equivalent plugs have on occasion caused failures. Similar heat range does not guarantee identical physical dimensions.

You can use either plug successfully, gapped at .030" to .040".

The only thing that is guaranteed is that others may disagree.

 

[1946Zephyr]

Re: '53 super fastwin....J4C or J6C?

Yea, my Seloc manuals call for a J4J in that motor, but I, myself would run a J6C. The J4C in my opinion isn't quite as good as the old J4J's. I happen to have a couple old 40's with the old J4J's in them and they run fine. I once had some J4C's in an old 5.5hp Jonson years ago, and that booger would only get half the rpm's that it was supposed to get. I put some old J6J's in it and it darned near run out from under me. LOL

 

[kfa4303]

Re: '53 super fastwin....J4C or J6C?

I like running the Champion plugs too. Not only are they what the factory recommends, but they seem to be readily and widely available at virtually all auto stores and their code is easy for me to remember. Some of the other bands have codes that look like alphabet soup. I would go with the J6C if you think you'll be doing a lot of trolling other wise the J4C should be fine. Cheers!