Grey Spark Plug Scum

1975 135HP Johnson

I pulled the plugs today and notice some odd coloration on two. The plug from cylinder 2 (top port) was coated with a thin layer of a wet silvery-grey scum. Plug 3 (bottom star) had some too. But the other two plugs were just wet.

I am still going through all the troubleshooting with the engine (setting timing, link-n-sync, etc.) but I think I've got it tuned okay now and ran it for about 20 minutes at idle and WOT.

Any ideas on why these two plugs would look different and if this color indicates anything I should panic over at this time?

Re: Grey Spark Plug Scum

You may want to run acompression check, I am pretty sure that indicates water is getting into those cylinders.
If you find compression is down on those 2 you will need a head gasket.
100+ and within 10% is good.

Re: Grey Spark Plug Scum

the gray may be water leakage.run comp chck ,has it been overheated that ya know of ,may have blown headgasket.may have exhaust gasket leak.
you should have comp in 90s that not unusual.but your going to be looking for uneven numbers .on the cyls that have the low numbers,if you have low readings you may want to get you a manual,
repost after comp check .
good luck

Re: Grey Spark Plug Scum

A grey insulator, maybe blistered, is a sign of overheating, maybe from loose plug, lean fuel, wrong heat range or bad timing.

Re: Grey Spark Plug Scum

HighTrim said:

A grey insulator, maybe blistered, is a sign of overheating, maybe from loose plug, lean fuel, wrong heat range or bad timing.

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The grey scum is on the outer electrode not in the insulator. To me, it looked like water mixing with oil but I didn't want to think that I was hoping someone would come up with something less tragic.

Re: Grey Spark Plug Scum

Randyg123 said:

The grey scum is on the outer electrode not in the insulator. To me, it looked like water mixing with oil but I didn't want to think that I was hoping someone would come up with something less tragic.

Click to expand...

Ah sorry, read it wrong. I would guess an exhaust gasket then, head gasket, or fuel that has been compromised with water somehow. A compression test would help narrow down the problem. A gauge can be bought extremely cheap nowadays at any auto parts store. Let us know how you do. Changing the gasket, if that is the case, is not as bad as you might imagine. If it is your first time, I would reccomend an OEM service manual though.

Check and make sure though that your built in tank is not letting if water, siphon some into a clear jar and let it sit over night, then inspect in the morning.

Re: Grey Spark Plug Scum

Comprssion was good (according to mechanic that tuned up the motor) before I started messing with it. Looks like now I will have to buy a compression guage. Pretty soon, I'll have all the equipment to be an outboard mechanic Is there any significance to the two cylinders involved? If they were both on the same head, I could see that I blew a head gasket during my repair attempts. But both heads, in one day Does this point to the exhaust gasket?

I only ran the engine for about 20 minutes in a river going back and forth between WOT and idle. WOT was in cycles of 3-5 minutes each time, max. Overall majority of the time it was idling because I had a very long 'No Wake' zone to get through. One thing I always do when I get the boat on the trailer is pull the gas line of the engine and let it run at fast idle until it runs out of gas. Is this a good or bad thing to do?

Re: Grey Spark Plug Scum

When you say 'messing with it', what were you doing exactly?

There is no reason to run the carbs dry, use a fuel additive instead, you are running the risk of running cylinders without lubrication, which is bad. It is different on a 2 cylinder.

Re: Grey Spark Plug Scum

HighTrim said:

When you say 'messing with it', what were you doing exactly?

There is no reason to run the carbs dry, use a fuel additive instead, you are running the risk of running cylinders without lubrication, which is bad. It is different on a 2 cylinder.

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Here is what I have done, keep in mind that when I started, I knew absolutely nothing about two-strokes.
Had a standard tune-up and lower-end / water pump service
The motor still would not idle or start
With the help of everyone here I troubleshot the ignition and all was well but replaced all four coils.
Tried to do a link-n-sync but was having a lot of problems (learning-curve).
Took the boat out and it ran fine for about 15min and then was only able to run at low throttle (shut off if I throttled up).
Found out that my engine was a 1975 135HP and not a 1973 85HP
Got a service manual and was able to do a link-n-sync and set timing close to where it should be.
Then got the boat wet to test everything as I described above.

I had the plugs out when I was working on the engine and did not notice the grey crude. But once I had the WOT timing set, I did not have the plugs out again until now. So I was still playing around with setting the timing and fine tuning the linkage and all. At this point I was running the motor partially submerged (up to the cavitation plate) in a tub to get back-pressure on the motor. Since I only had the motor partially submerged, I kept earmuffs on with running water to make sure I wasn't sucking air into the water pump. Could this setup have screwed something up?

So the plugs were only exposed to this operation and the the short run on the water to test. I asked this before but is there any significance to the two cylinders involved (2&3) that could help narrow in on the issue? Since they are not on the same head, what do they have in common?

Re: Grey Spark Plug Scum

If you have not replaced the plugs since you started, why don't you try new ones and see if the problem continues.

If the problem is that you are running lean or that the timing is off, the grey may be aluminum and that is not a good sign at all.

The issue of the compression test is no longer an option; you have to be the eyes for all of us.

It would have been better to have the timing set by a mechanic with the training and tools. Why did you reset it?

Re: Grey Spark Plug Scum

ezeke said:

If you have not replaced the plugs since you started, why don't you try new ones and see if the problem continues.

If the problem is that you are running lean or that the timing is off, the grey may be aluminum and that is not a good sign at all.
The issue of the compression test is no longer an option; you have to be the eyes for all of us.
It would have been better to have the timing set by a mechanic with the training and tools. Why did you reset it?

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Yeah, i was worried the silvery stuff could be aluminum. But it seemed kind of pastey-looking and not so much metallic.

As for the timing, just following advice found here.
I checked the WOT timing using the method on this site and it appeared that my timing was set way advanced but could not retard it more than about two degrees. Then someone told me that the method posted here did not apply to my engine and the four-degree adjustment factor did not apply, so I put the timing back to about one degree retarded from where it was originally. Just to be sure the timing was not set advanced. As for running lean, would my plugs still be wet?

Re: Grey Spark Plug Scum

Regarding the wet plugs, that is either water or unburned oil/fuel mix, combining with some material from the engine. If the motor is run under load at cruising speed, the amount of residue will be less.

The timing is critical and should be set according to your service manual. See page 4-16, and page 4-4.

The timing of your engine should not change during normal operation, and should only require adjustment when the system has been removed and/or replaced.

Re: Grey Spark Plug Scum

Will pick up a compression tester today and test tonight. I am hoping to get everyone's comments now as Friday night may be tough time to get advice. So I am trying to get any tips or comments now and be ready to examine as much as possible tonight.

I was planning on going fishing this weekend but now fear I will ruin the engine (if not already) if I use it even lightly. If it is a blown head or exhaust gasket, is there any problem with continuing to use the motor for a while? Other than performance issues, I am not so worried about that. I just don't want to screw it up permanently. If it is a piston head dissolving, is there any other symptom to look for that an untrained eye can easily see? If the problem is timing, shouldn't the symptoms be noticed in all cylinders?

Thanks again for all the help so far (and to come).

Re: Grey Spark Plug Scum

You might also get a gray-white looking substance from oil and water mixing in the cylinder if the cylinder(s) did not fire.

Normally, water in the cylinders turns to steam if there is good fire, that cleans the spark plugs and leaves them looking good.

The compression test may let you know if there is a blown head gasket, but will not tell you if the exhaust gasket is leaking.

Re: Grey Spark Plug Scum

ezeke said:

You might also get a gray-white looking substance from oil and water mixing in the cylinder if the cylinder(s) did not fire.
Normally, water in the cylinders turns to steam if there is good fire, that cleans the spark plugs and leaves them looking good.
The compression test may let you know if there is a blown head gasket, but will not tell you if the exhaust gasket is leaking.

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Well they are obviously not getting steam-cleaned but then I didn't run the engine much so maybe not enough time to clean the offended cylinders.

Is there any way to check for an exhaust gasket leak?

Re: Grey Spark Plug Scum

Compression Test Data

Here are the results of my compression testing.

Pulled all sparkplugs and then grounded them out.
Kill switch on
Earmuffs on

About 4-6 cranks each

#1 - 150
#2 - 160
#3 - 155
#4 - 145

When brought in for a tune-up the mechanic reported all cylinders around 150 so these values agree with his.

Rinsed off the plugs with some carb cleaner last night so they were kind of clean. Re-installed plugs and started the motor and let it run at about 750rpm for 10-15 minutes. Enough to really annoy the neighbors. Shut off, feel heads and they were just warm, not hot.

Pull plugs and each were wet with fuel.

Ran compression test again:
#1 - 150
#2 - 155
#3 - 155
#4 - 140

Note that #2 and #4 were a little lower, but barely. Not sure if this means anything. I checked the compression in #1 first, then when I went over to the port side I saw some smoke or steam coming out of the #2 cylinder before I attached the gauge. I cranked the motor a few more times to see if any other cylinders exihibited this but none (including #2) did show any smoke/steam coming out of the sparkplug hole this time.

Well......please now just tell me that all that panic was for nothing and all appears to be just fine Please

Let me know if there are any other observations that could be critical to your ideas. But do it soon my memory is quite short. Thanks again!

Re: Grey Spark Plug Scum

i wouldnt run it long till you can verify timeing .

Re: Grey Spark Plug Scum

Thanks, I will check the timing again. But when I did so last week, I used the WOT method recommended by Mr. Reeves and was then told by another member that this method's "+ a factor of 4" does not apply for my engine and to accept the reading as is. I did not get any replies to the contrary after that so I accpeted that as probably true. So right now, my timing should be at about 19-20 at WOT. This is about where it was when I started. The idle setting was at 5.

You can check out my thread on my timing issue to see the issues I had with that. I do not know how to add a link to a post so you will just have to search my threads to find it.

Does anyone know how I determien if there is an exhaust gasket issue?

Thanks again for your assistance.

Re: Grey Spark Plug Scum

Usually an offending exhaust gasket would cause the motor to run poor with the cowl on, since the exhaust gases are changing its air/fuel ratio. There are other tricks as well, but before getting into that, I believe you should start at the timing.

Timing on these outboards is critical, and personally, I would not continue until it is set perfectly. You have to remember, alot of the guys on here are trying to help, but are not marine mechanics, so you have to take their word for what it is worth. I have personally often messed up, not intentionally, but like I said, I am not a mechanic. Guys like Joe Reeves however are highly knowledgeable OMC mechanics, and if he gives you guidelines for setting your timing, I personally would listen to him. When in doubt, consult your OEM Service manual, it will not lead you wrong.

Re: Grey Spark Plug Scum

Thanks "High".

I have tried to look at all the advice i get with an open mind and greatly appreciate all comments.

In fact, I was waiting for some folks chiming in if the comment that (the WOT method would not apply on the engine I have) was incorrect. But no-one did so I figured it must have been a good comment. I went back to look at the post and the comment was that the method is "not accurate for the PP4 ignition system on your 135" which is all greek to me. I don't know a PP4 from a CDI or even if they are the same thing.

So now i am not going to do anything until I get some comments on this issue. Should i start a new thread specifically addressing it?