Mystery dry spark plug

[Joolz]

1997 Merc Force 120 ELPT running Champion surface plugs UL77V (manual calls for L76V). Came across an odd issue with one of the spark plugs. When in doubt, ask away, so here I am.

Got this boat last week, yesterday did a compression test and all strong within a 10% margin. But as I removed the plugs I noticed some inconsistencies with their condition, took pics for safe keep. What caught my eye was how wet/grimy #2 from the top is, and how dry #4 is (bottom plug).

Here's the puzzling part. Today I cleaned the plugs and gave them a very light sanding, installed, run the motor for a couple of minutes and removed the plugs to do an inspection. All the plugs except for #2 had a ring of unburned fuel in them, which is what you'd expect. But #2 was dry. Swapped the plugs, run the same test and once again #2 was dry. Thinking that maybe it didn't have a spark I swapped wires (wire and module). While I had the wire harness for cyl 1 and 2 out, I run them past the Ohmmeter and both checked out, as well as continuity. But I swapped them around anyway, since they're the same part number and same length. Run the motor for a bit and once again the plug pulled from cyl #2 was dry.

Fearing that I had a cylinder that wasn't firing I run the motor and pulled the top plug's wire and noted the immediate reaction of an engine running on 3 cyl. Run the same test but this time I pulled the wire for the questionable cylinder. Motor immediately choked as expected. So the cylinder does work, it is kicking. Should also point that while running it sounds beautiful with no signs of anything being amiss.

So what gives? Why is that plug dry after running the motor for a few minutes while all the others have a ring of unburned fuel in them? Note the pic below and how wet #2 is, which is the complete opposite of the results I’m getting. May be a non issue or it could point to something else, like not enough fuel getting into the combustion chamber or perhaps timing that’s off?

 

[tommarvin]

Thats right mercury made force for awhile.
Good for you to do the cylinder drop down test, its weird that cylinder was working
The plug cant be dry if the cylinder is getting gas and firing.and the drop down test.
Be sure to put everything back to where they were before you started swapping .everything
It sounds like its just a clean/cleaner burning cylinder, than the rest, not sure.
You should do a decarb, 16 oz seafoam mixed with 32 oz of fresh gas, run it all through the motor,see if that helps.remove the outlet side of the squeeze/primer ball, we had to add 16 inchs of hose to the fuel line to go in the one gallon jug of decarb fluid.

 

[Joolz]

Decarb job huh? Sounds like a good idea and a good time to do it for a fresh start with the new owner. So to confirm I don't need to add premix oil since it's got all that Seafoam already huh? Even if it doesn't do anything to answer to the mystery dryness of the plug it could help clean the lines/carbs a bit.

One issue though: I don't have an external gas jug with the Mercury end that I could connect to the motor. I do have a Johnson one but I haven't looked to see if these are compatible. Or is there another way to feed the cocktail to the motor?

 

[GA_Boater]

If you want to get top dollar, you better add Seafoam to premixed fuel or the motor will seize from lack of oil. Seafoam is not lube, it's an alcohol based fuel additive and has no lube qualities.

Just dunk a hose in a gas can with a gallon of gas mixed 50:1 and the amount of Seafoam used for decarb.

 

[Joolz]

Ahh, I missed the part about using the hose from the primer bulb. K, will look into that then, even if I have to add an extension. However the recipe for the cocktail isn't clear to me. Should I add a can of 16oz Seafood (450ml) to a litre of already PREMIXED gas or a gallon of gas? The amounts above don't match up.

Litre of course being close to 32 of the American Oz. While a gallon is nearly 4 litres. Suppose the first is the correct one, wouldn't want to sit there all day waiting for this mix to run if indeed the recipe calls for a gallon of gas.

 

[racerone]

Does spark jump a gap of 7/16" ( 1 cm ) on each lead , yes or no.

 

[Sea Rider]

Is the lower plug firing with intensity when removed and tested while grounded to powerhead ? From my experience not all plugs remains same whether clean, fouled, whichever, worse if being a 2 strokes engine. Besides all 3 carb jets are not same, usually one carb has smaller jets compared to the other 2, a matter to check the Spare Parts Book.

If that applies for that engine, probably the carb with the less jets works with the cylinder along plug with issues ?

Happy Boating

 

[Joolz]

To clarify I was unable to do a spark test. Couldn't comfortably pull back on the boot to expose the wire. Thought if I forced it any more that something would break. So instead I swapped the wires around (top with the one below it). One of the ways I could test for a spark would be to add a piece of wire from the lead and then bring this close to the engine's block. But a bit scared this could cause me a nasty shock or that having a piece of wire connected that it could compromise the size of the spark.

Still not sure if there is an actual problem here, seeing how well the motor starts and runs (in place as I haven't had the chance to bring it to the lake yet) and how good the compression is. Mostly curious as to what could be the reason for the spark plug to be so dry (again, used different spark plugs on that cylinder with the same result), while its counterparts have this ring of unburned fuel around the rim.

Another thing that had me puzzled is this: how do you test a surface plug? With a J style plug it's easy to use an Ohmmeter and test them. With the surface I couldn't get a reading no matter where I tried to connect the end of the needle.

 

[Faztbullet]

That plug looks rusty to me....possible lower seal problem.

 

[Joolz]

Good spotting, in fact there was a good amount of rust on the other end of the plugs, as well as a bit inside the rubber boot (the metal tabs that connects to the plugs). I have no idea how this relates to the lower seal but if you'd care to expand on that I'd appreciate the information - new to boating, here to learn.

In case this is related, upon checking the leg oil, I noticed that it may have a bit of water intrusion as the oil didn't appear to be in its normal dark brown colour. I will know better once I fully drain the oil prior to putting the boat away for winter (fingers crossed I'll be able to take it out once before our horrible weather prevents it).

If there is suspicion I may have a bad seal somewhere I'd like to know where to look and have these addressed. The plan is to keep this boat for the long haul, so will do any necessary maintenance.

 

[Faztbullet]

The lower crankshaft seals can leak allowing water to be drawn in thru seals....this steams cleans the plug and leaves rust on electrode surface.