Champion J4 Sparkplugs for JW, Lightwin, and H Models

[aquasonic]

Last Spring while working on a 1963 Johnson JW, I noticed that it had Champion J4 sparkplugs. All of the other older motors that I’d seen of this type always had J6 sparkplugs.

After doing a little research, it looks like OMC started using J4’s in the 3 HP in 1960. Now, I wonder why the change was made. These motors run on the hot side, so with the J4 being a cooler plug than the J6, this makes sense. Was it the improvements in 2 stroke outboard oil that made the cooler plug possible. 30w automotive oil was recommended and commonly used in the 50’s, but modern 2 stroke oil is probably closer to 20w, and is supposed to be less prone to fouling plugs.

The H series motors, with the integral carburetor that includes the exhaust bypass, are notorious for boiling the gas in the carburetor and causing a vapor lock which prevents the motor from starting until it cools down sufficiently. I have an HD-25 that is super reliable, but does suffer from this vapor lock after running at WOT, then going right to idle, it can stall. These motors had a factory recommendation of J6 sparkplugs, and heavy 40W automotive oil.

This season, I’m going to try J4’s in the HD-25, with TCW-3 2 stroke oil, and see if they work without fouling. My hope is that this helps the vapor lock problem. Your thoughts?

• This topic was modified 3 years, 3 months ago by aquasonic.

[Buccaneer]

Have you ever noted the page from the service manual regarding vapor lock on these motors?
Kind of interesting…….

Prepare to be boarded!

[outbdnut2]

Now you’ve got me thinking …….Did the switch to J4 plugs coincide with the 1959 addition of thermostatic cooling on 5-1/2 HP and larger? Of course the JW being 3 HP did not get a thermostat.

Another thought on the matter – I always thought that spark plug heat range had nothing to do with how hot the engine ran, with heat range only controlling the temp of the spark plug tip to prevent fouling and degradation of the firing tip and it’s insulator. Colder plugs conduct more heat from the tip to the cylinder head, so should make the engine temp run slightly, but maybe not significantly hotter. Is this right?
Dave

• This reply was modified 3 years, 3 months ago by outbdnut2.

[aquasonic]

Thanks Buccaneer. I have seen that page from the service manual. My HD-25 is from a much earlier serial number 570699, and does not have the clean out screw where a small section of brass rod can be inserted to minimize the fuel capacity within the carburetor. One could probably be drilled, and tapped there though.

Regarding the spark plug model vs. motor temperature, I’m not sure how that relationship works.

[Author]

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