Spark Plug Question

[bh357]

1963 Merc 1000.

The manual calls for J4J plugs. Currently, the motor has RJ6C plugs, which were in the motor when I got it. The motor seems to run good.

What are the significant differences between these plugs? Does anyone know why the other plugs would've been subbed? I will be replacing the plugs, due to their unknown age. Should I replace them with the proper J4J plugs, or with another set of RJ6C plugs?

Thanks.

 

[tashasdaddy]

Re: Spark Plug Question

j4j was replaced by j4c, j6c is a hotter plug. if this motor is new to you do a compression test before spending any money. just tells the health of the guts.

 

[Texasmark]

Re: Spark Plug Question

R should indicate resistive noise suppressing element and C for copper tip. Otherwise it should be like a J6J and as tas responded runs hotter and that should be to burn off contaminants on the plug tip better than the J4J would.

Beins I'm a converted (to) Sea Foam user, if you run it in your gas (all the time) it should keep your combustion chamber clean and significantly reduce spark plug fouling.

I can't believe the stuff. First in the outboard and it cleaned it up, then the weedeaters I was going to throw away, then the hay truck that's 15 years old, and just the other day I put it in a '64 diesel Massey Ferguson tractor that was blowing a lot of black smoke (both crankcase and fuel on this baby) and it not only cleaned up the smoke, my oil pressure has now risen up to where it belongs........saved me an overhaul.

Mark

 

[bh357]

Re: Spark Plug Question

Thanks tashasdaddy and Texasmark. Based on your responses, a J4C or J6C would be acceptable in this motor. I'm guessing that at one time the plugs were replaced with the RJ6C plugs to help with fouling (possibly extended idling, etc...).
So, does it sound like a good plan to replace them with J4C (or RJ4C), and keep an eye on them for any fouling?
Tashasdaddy, regarding the compression test, I'll be doing that sometime in the not too distant future. The motor does seem to accelerate quite nicely, planing out in about 3 seconds (14 foot fiberglass, weighing about 700 lbs without motor, fuel, or battery), with 2 people on board, and about 3 gallons of fuel. The prop on it right now is a 17 pitch.

 

[Texasmark]

Re: Spark Plug Question

May have had to with availability like WM only stocks certain types and maybe the guy didn't want to shop elsewhere (if he was shopping there in the first place).

Personally don't think they had anything to do with performance; the resistor is for ignition noise suppression in radio (sonar) equipment and the copper is for increased element life.

I always like to run a hotter plug and have never been sorry so I'd opt for the J6J.

3 seconds on a hole shot is super. If you like your top end baby, you're there.

My 2c

Mark