# Fouled plug in one cylinder, 18hp Johnson



## kroppe (May 7, 2000)

Noticed when putting in new plugs, that the bottom spark plug is a little black in my 1965 18hp Johnson outboard. I'm not terribly worried, but was wondering what might be causing it. 

As far as I can think of, the causes could be bad rings, scored cylinder, ignition problem or a fueling (carb) problem. 

I would like to rule out the carb, since I think it's a single barrel therefore there wouldn't be any fuel distribution issues, or would there be? 

As far as ignition goes, would a bad plug wire cause plug fouling? What about a bad coil? I think a bad coil would affect both cylinders equally.

Maybe a leaking gasket on the bottom cylinder? I'm not sure how all the gaskets are put together on this engine, so maybe you experts can advise me on this one.

I will do a compression check to see if the rings are OK and report back.


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## ESOX (Nov 20, 2000)

Just one coil on there?


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## steve ypsi (Nov 24, 2002)

I replaced my plugs years ago, it was a 2 cyl, well I was running on one cyl, of course I checked the plugs last since they were new, one was no good brand new and really thru me off, I would try reversing the plugs and see if it fouls the one that was running right if you have some muffs to run as hose on. A cheap compression gauge will tell you a lot, if the good plug is a lot higher than the other it might be a ring or gasket but I had a friends that I was checking that was running bad 6 HP, it read the same on both cyl's on a compression test and I pulled the head and the gasket was broke between the cyl's


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## Banditto (Jun 9, 2000)

Yep good things to check from previous posts. Fouled plug could be any number of things, but bad coils will definetly foul a plug quick. 

Things that cause a plug to foul are bad or wrong spark plug, improper gap, bad fuel mixture, bad coil or wire, carb out of whack, cylinder losing compression, leaky valves, probably a couple more things that I missed.

The coil on my old Johnson was bad and it was a bear to replace. You have to remove the flywheel to get to it. But yours is probably newer than mine (90'ish) and it may have electronic ignition which would mean the coil wouldn't have to be mounted under the flywheel like mine was.


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## burbotman (Feb 20, 2001)

I took my boat out with a 40 hp 1974 Evinrude today, engine ran fine for a while.Then when I made it on the way back to the launch, it started running like s#$%.I thought it ran out of gas.I switched over to the other tank, same thing. I then pulled the plugs out and examined them, one looked fine but the other one, looked like it was coated with chocolate milk. I changed the plug, then I proceeded to make it back to the launch. I then pulled it out of the water, and after examining the motor, noticed an oily substance around the head gaskets. I was wondering where I can get the head gaskets and head bolts at?Thanks....any help will be greatly appreciated...


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## ESOX (Nov 20, 2000)

A little more info in your profile other than "Michigan" might prove helpfull........


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## burbotman (Feb 20, 2001)

Thanks, Esox. I've already found a place that I can get them at.


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## kroppe (May 7, 2000)

I did a compression check today on the motor. It was stone cold from winter storage, hadn't been touched since it was fogged. There was some fogging oil in the cylinders. I got 72 psi on one cylinder and 75 psi on the other. I pulled the cord about 12 times for each cylinder. I have read a little bit on several sites and it seems 75 psi might be OK, but 90 psi would be better. 

First question, is 75 psi low compression, low enough to do something about? 

Second question, what else besides head gasket and rings could cause low compression? I have no oil seeping/weeping out of the head gasket, so I don't think there is a problem there.

Since I have close to the same compression reading on both cylinders I am thinking (hoping) it's not the rings. The motor really runs quite well, I'm just chasing down a single fouled plug and trying to locate the source of the problem. This might something that falls in the category if "if it ain't broke don't fix it." 

The motor starts and idles well, and pulls to max rpm well, I'm just trying to get that last 1%-2% out of it.


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## TONGA (Jul 22, 2002)

Krope are you shure you have a good gauge,, 72 psi sounds very low to me,,,I fix a lot of outboards and let me tell you that is low,,,My old beat down 55 horse Evenson has better than 120 on boath jugs,,,
I cant imagine your boat moter running well with only 72 psi
My old but nice 5.5 evenrude fisherman still has 120+
Las year i tore down a engine that had only 63 psi on one jug,,and found it had burnt a hole right through the piston!!
The fact that you are getting the same reading on #1 and #2 and you say it runs well leads me to think you have a bad gauge !!
One simple way to look for a miss-fire is to run the engine with the cover off at nite time,,this will reviel any bad wires,conections,cracked plugs ect....you will see the spark leaking out


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## kroppe (May 7, 2000)

Tonga, 

I rechecked compression again and this time I got 115/120. Now that's a lot better. This time I used the electric starter to turn the motor instead of pulling the cord, plus I made sure the gage was screwed in tight against the o-ring. 

I also checked for spark and there was lots of strong spark on both plugs. So I guess if there is a problem at all, it might be a fuel distribution problem leading to a rich cylinder. How could that be possible with a single barrel carb? 

Dang I'm pysched for Sunday, launching from Wyandotte. I checked the trailer lights and boat lights, everything is A-OK.


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## TONGA (Jul 22, 2002)

you did not say what year your engine is,but if it is a single carb feeding boath jugs I don't think you have a fule delivery prob.I would still try running it in the dark,,and if you have something like a hairline crack in a spark pug, it will show itself by the visable spark,,this is a quick and easy to do and can save you a big head-ache on the water.
I don,t know how bad the plug looked when you pulled it out,but some collor it normall in a outboard..a little brown is fine just so long as it's not real wet.
One think to watch for is a plug that looks to clean,you know like brand new shinny looking,,that is not a good sign,,9 out of 10 times it means water is getting into the jug...nothing cleans a plug like water in a running engine
PS I like your new numbers a lot better 115 and 120 thats about the norm for a healthy moter,,,,72 psi man that sends a chill down my spine


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## kroppe (May 7, 2000)

Tonga, 

I put new plugs in it so there aren't any cracks. I checked the spark in the garage with the lights off and there was excellent strong spark. I'm putting her in the water tomorrow so we'll see how she runs.


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## TONGA (Jul 22, 2002)

Ok but new dose not mean good,,my buddy mike got a brand new cracked plug last year,,he was pulling his hair out till I found it for him!! it's not likely but it happens,,I have seen guys put $100 switch boxes,and $30 coils on their engine only to find it was a cracked spark plug or a bad boot on a plug wire..
upset yes they were, no refund on electrical parts!!
I am sure you are probably in fine shape now,,all I am really trying to say is when you have a missfire,start at the plugs and work your way back,,,we always think the worst but most of the time it's the cheep stuff that go's bad even if it is brand new


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