1973 Johnson 25HP Sparkplugs

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rednecknproud21
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1973 Johnson 25HP Sparkplugs

Post by rednecknproud21 » 13 Aug 2016, 04:29

I'm confused as what sparkplugs I should be using. So many websites say different ones. The motor currently has J8C champion plugs are these the correct ones or should I use something else? Any insight would be helpful.ImageImage

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Pappy
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1973 Johnson 25HP Sparkplugs

Post by Pappy » 13 Aug 2016, 09:46

J4C is the correct plug. The J8C is too hot for that application.
This comes directly from the book...no "Well, Bubba uses this one!"

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rednecknproud21
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Re: 1973 Johnson 25HP Sparkplugs

Post by rednecknproud21 » 13 Aug 2016, 10:47

Pappy wrote:J4C is the correct plug. The J8C is too hot for that application.
This comes directly from the book...no "Well, Bubba uses this one!"
Thanks pappy I don't know why some sites list the j8c. That's what was in the motor when I bought it I will pick up some of the right ones when I get off work.

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richg99
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1973 Johnson 25HP Sparkplugs

Post by richg99 » 13 Aug 2016, 13:52

Hay, Pappy....how's about explaining to me, and anyone else who is as dumb as me, when you need a "hot " plug and when you don't......

richg99

p.s. my 50 hp '96 Johnson has to run at idle 90% of the time due to the "no wake " limitations on my main lakes..

Sinkingfast
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1973 Johnson 25HP Sparkplugs

Post by Sinkingfast » 13 Aug 2016, 16:01

The easy answers can be found in a book. Easy, simple answers are good for persons who are not into wrenching or tuning ones equipment to real world usage. The op just wants the motor to run at factory specs.

Now some of us tune our boats to our normal running conditions. I run 4oz per gallon, up the main one and run one or two level hotter plug. What is the op going to do with that info but be confused.

...don't mean to answer for Pappy...
2 stroke every time I can
4 stroke only when I have to..I don't like boating with my lawn mower

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richg99
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1973 Johnson 25HP Sparkplugs

Post by richg99 » 13 Aug 2016, 16:07

OK I'm confused...

" I run 4oz per gallon, up the main one and run one or two level hotter plug."

"up the main one" ??????????

richg99

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rednecknproud21
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Re: 1973 Johnson 25HP Sparkplugs

Post by rednecknproud21 » 13 Aug 2016, 16:09

I would like to know why you would want a hotter plug because maybe someone was trying to hide something like bad cylinders or they did a heck of alot of idling for trolling.

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Sinkingfast
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1973 Johnson 25HP Sparkplugs

Post by Sinkingfast » 13 Aug 2016, 16:34

richg99 wrote:OK I'm confused...

" I run 4oz per gallon, up the main one and run one or two level hotter plug."

"up the main one" ??????????

richg99
I have drills and 43 years of tuning everything that runs on gas, propane or oil. Boilers, weed whackers, saws and dirt track autos. Sometimes I use local terminology not used here. Sorry..

Did you eat something bad for you yesterday??
2 stroke every time I can
4 stroke only when I have to..I don't like boating with my lawn mower

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Pappy
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1973 Johnson 25HP Sparkplugs

Post by Pappy » 13 Aug 2016, 20:18

There is absolutely no need for a hotter plug in that engine. Can you run a J6C? Sure. Did I just contradict myself? In most of the older engines there was a recommendation for one step hotter plug for prolonged idle. This engine may or may not still fall into that category. A J8C? Nope.
The answer to prolonged idle is to make sure your thermostats are working properly first and foremost.
The engine needs to be good and warm at idle. If you are lucky enough to have TNT (tilt and trim) you can also
"tailor" your idle mixture some by trimming up or down while idling. Trimming up will lean the engine for cleaner running and vice versa. Try it. You will notice your RPM going up in about 10-15 seconds after trimming up and vice versa.
The issue you get into when going too hot with a plug would be piston damage. Not worth taking a chance when the other
controllable factors are quick and easy.

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rednecknproud21
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Re: 1973 Johnson 25HP Sparkplugs

Post by rednecknproud21 » 15 Aug 2016, 18:13

Thank pappy

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rednecknproud21
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Re: 1973 Johnson 25HP Sparkplugs

Post by rednecknproud21 » 17 Aug 2016, 20:44

I went out today and ran through 3 gallon of gas the new plugs in the right heat range made a heck of a difference thanks so much for your help

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Pappy
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Re: 1973 Johnson 25HP Sparkplugs

Post by Pappy » 18 Aug 2016, 09:11

rednecknproud21 wrote:I went out today and ran through 3 gallon of gas the new plugs in the right heat range made a heck of a difference thanks so much for your help

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You're welcome and thanks for getting back to us!

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