Enlighten me about spark plugs

[thepetrolist]

Hi guys

Spark plugs have always been kind of a mystery for me. So simple yet so hard to understand. I guess many of you feel the same way about something really simple.

Anyway, today I got me some new spark plugs for a couple of my engines (’63 sportwin, ’60 JW16).

When I got the engines, they both came with J6J plugs. The technical specifications recommended to use J4J’s, but I didn’t realize that until I had already bought them. I just bought the same plugs that were already in the engine. Well, they were Bosch W8ECs but they’re still the same.
I installed the plugs and they both ran fine, so I decided to try some new plugs in my 1951 HD-26 since it too is running some really old J4J’s. Here comes the mystery part:
The engine started just fine with the new plugs, then started to run rough, didn’t rev, and finally died. Then I just couldn’t get it running again! I tried and tried and tried, but no luck. I reinstalled the old J4J’s and it fired right up and run really well again.
Note, new plugs were J6J spec plugs.

Can anyone tell me why this happened? Is the J6J so much hotter than the J4J that it simply wouldn’t run on it?
If so, why did the JW-16 run with those plugs but not the Hd-26? They are still pretty much the same engines.

Note no. 2, nothing but the plugs was changed. I ran fresh gas with fresh oil.
Oh, and I ran the exact same plugs in the JW as in the HD, I simply switched since I had bought too few (luckily).

Seems I have to go back to the store to get som J4J spec plugs now.

[frankr]

I seriously doubt the heat range (J4 vs J6) had anything to do with it. Something else is going on there. You mentioned Bosch. I am totally ignorant concerning Bosch numbers. They may not be exact interchanges. Maybe a resistor plug or a booster gap plug or something on that nature? Or I could be off base here. Also could be something as simple as different plug gap.

[The Boat House]

• This reply was modified 6 years ago by david-bartlett.
• This reply was modified 3 years, 9 months ago by The Boat House.

[legendre]

It’s possible that the hotter plug caused a slight overheating condition which resulted in vapor lock, stalling the engine. But otherwise, about the only way a too-hot plug stops an engine is by provoking a seizure or burning a hole through the piston. What did the new plugs look like, when you pulled them from the stalled motor?

BTW – was this all done in a test tank, or out on the water? Sure, these are water-cooled motors, but they can get a lot hotter ‘under the hood’ when sitting in still air – and that’s where the vapor lock originates.

Finally, for the most part, plug cross-references are general guides and some types / makes of plug just seem to work better in certain motors. As you might have noticed, NGK is my go-to plug – they’ve never done me wrong. And you should +never+ use any other plug (except for perhaps a Denso) in a Japanese motorcycle or car. However, the Denso u-groove plugs burn extremely well in the older Ford L6 motors, 240 cid, 300 cid.

I’ve never had good luck with Bosch plugs, except in BMW motorcycles and one particular moped. Champion are OK for lawn mowers, snow blowers, outboards / power equipment and.. Harleys. Otherwise, I avoid them like the plague for motorcycles or cars.

When NGKs are two bucks, and widely available, I just can’t think of any reason to mess with anything else, really. Maybe try a pair of B7S some time.. or B6S if you want a little hotter, B8S for a little cooler.

[pappy]

quote legendre:

It’s possible that the hotter plug caused a slight overheating condition which resulted in vapor lock, stalling the engine.

Talk about the need for enlightenment…….!!

[retiredoz]

Did you put the plugs that failed in the HD-26 back into the JW ? Just curious.

[thepetrolist]

quote Pappy:

quote legendre:

It’s possible that the hotter plug caused a slight overheating condition which resulted in vapor lock, stalling the engine.

Talk about the need for enlightenment…….!!

Yeah, well, I guess you’ve never been a newbie and never made mistakes?
And sometimes ones brain farts and can not for the love of something figure out whats wrong.

[thepetrolist]

quote retiredoz:

Did you put the plugs that failed in the HD-26 back into the JW ? Just curious.

Of course not.

[Randy in Tampa]

Try autoliite 303’s i’ve had good luck with them 😉

[chinewalker]

I have found that a great deal of the "this brand of plug works better than that brand" stuff is more to do with set-up of the ignition, quality of the fuel, and quality of compression. I have never seen a motor issue that could be directly attributed to plug "brand". I have seen many that were blamed on plugs that were actually related to poor quality fuel, incorrectly adjusted carburetor, poorly maintained ignition, low compression, leaky gaskets, etc. If your compression is good, fuel is clean and fresh, and your spark is hot, it’ll run on the recommended plug, no matter the brand.

[Author]

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