Was the motor idling for a while or was it running at power when the motor was stopped for the plug check?

What is model number of the motor? What is the age/quality of the fuel? If the fuel pump diaphragm has a hole/tear in it then it can cause raw fuel to be pulled into that cylinder causing rich fuel mixture fouling.

Thank you, both for answering this thread. I apologize for the delay; I was away from the boat and in Puerto Plata yesterday.

boscoe99: I had run to the town dock that morning and returned, a total of half-a-mile, but didn't run the engine hard. I took the photos out and photographed them after lunch, when the engine had cooled. I usually make that run to the village twice a day.

Robert: Engine is a 2009 two stroke, Yamaha 15MSH. Model #684KS1057789. Fuel is fresh. There isn't an outboard mechanic here... can I check the fuel pump myself?

Bry

Easy to check...remove fuel pump from motor with 2 bolts, but leave pump hoses hooked up...then pump the primer bulb several times...if pump diaphragm is bad then fuel will come out of the small hole/port on the back of the pump...if not then pump is fine. It's worth a check and a new pump is easy to replace and about $50...

Have you done a compression check?

Is the miss at idle or at power?

I would just clean the plugs, regap and install them in the opposite cylinder from which they were removed. Then get the motor warmed up and run it at power for a half mile or so. Shut the motor down and then look at the plugs? Same as what you are seeing now or is there a difference?

Then run the motor at an idle for a bit of time. Then recheck the plugs.

What oil brand are you using?

If you have new spark plugs (old but still new), replace them. You can have a damaged plug. Run the engine (either on muffs or in the water) in a DARK AREA. Your looking for any arcing around the spark plugs.

Also check the spark plug caps to the igntion wire that their tight. Its possible on the last plug change, the cap got dislodged from the wire snd there's some serious internal arcing going on.

I can't tell from the parts fisch; http://www.boats.net/parts/search/Ya...CAL/parts.html if the plug cap unscrews from the wire or not(doesn't appear so).

It IS a common occurrence on some motorcycles with removeable caps.. (the caps, if removeable simply screw on clockwise. If its black inside there with arcing, cutting off about 1/4" of the wire and re-installing should fix it).

Can you pull both plugs, ground them to the block still hooked up to the ignition wires and have someone crank the engine over checking for spark.

You can do this with both the old and new plugs. That bad plug is likely fouled and will have noticably less spark.

If you have a "spark tester", that would be the BEST way to rule out the ignition. Check you connectors at the coil and clean the ground to the coil.

And lastly, did you notice the misfire right after the last spark plug change or ANY (even slight) misfire when last working on it?

It's been four days since I swapped the plugs and I just had them out again to have a look. They look pretty similar but the plug in the lower cylinder has lost that tan color on the insulator and is now black. It is not bad but I'm hazarding a guess here, that both plugs are functioning and, as others have suggested, it might be a coil or a sticky piston ring in that lower cylinder. While I had the cover off, I removed the spark plug cap on the lower cylinder. No evidence of an electrical arc but the spring-like clip is not exactly a snug fit.

boscoe99: No compression tester, I'm afraid. The oil here is Jayco... not the best but it carries the manufacturer's designation.

Scott: The plug cap just pulls off and there is a spring or coil of wire that slips over the end of the plug. There is no tension, it doesn't snap on. Just slips on and is held in place with the rubber cap. But no signs of arcing! It is difficult to remember when the issue started. I did have to replace my gas tank several months back and while waiting for the new one to arrive, used a gallon jug. Of course, one night it rained like hell and the next morning, the engine died. I blew out the hoses, cleaned the fuel strainer and changed the plugs. It started up fine after that. But I can't remember if it started to rattle after that incident. I'll conduct the other tests you mentioned and get back to you with the results.

Thanks guys... really appreciate the help.
Regards,
Bry

Even the spark plug cap with the spring style inside should fit snug (W/O the help of the boot). You should be able to gently squeeze the connector just a touch to fit tight onto the spark plug.

Does it appear as if the spark plug cap unscrews from the wire or is it ONE PIECE(a molded unit)?

I don't think this would apply, but some spark plug cap wire ends, simply slip into the end of the wire, and another lip pokes into the side of the wire(to help keep it in place). If you can gently push (maybe some silicone on the wire) the cap back farther onto the wire, you should be able to see / check for arcing/a bad contact, etc.

.

If you got water in the carb and did not clean it out it can cause problems.
I guess this motor has only one carb, but there could be water still in the bowl.

Not sure it would cause plugs to turn black though

Townsends: Yes, the cap is a molded unit that just slips on. The clip to the plug is a coil of wire, the end of which is pressed solidly into the core of the wire. The coil is a loose fit but there is no evidence of arching.

99yam40: I'll check out the bowl again. I loosened the screw when I got water in the system and everything seemed fine and the engine started. I figured it was ok. But I'll take another look, just to be sure!

Thanks guys,
Bry

You did not say you drained the carb , so I just wanted to make sure it was attended to.
The drain screw should get most of it out of the bowl, but maybe not all.
And there can be water hiding in fuel pump and lines that ends up in carb down the line

I took another look at the carb this morning before the wind got up. The drain screw has a hole drilled in it so you don't have to remove it totally to drain the bowl. That's what I did last time.

This morning, I removed the screw and squeezed the prime-bulb. It looked like good old gas coming out: no dirt, no water.

But I did notice that, BEFORE I did this, the bottom of the carb was wet as was the bottom of the air filter housing.

I checked the plugs and the top one, the one that was originally in the bottom cylinder, was cleaning up nicely. The bottom one is still black. So I guess it isn't a bad plug! Too bad... that would have been an easy fix.

I'll get back at it tomorrow... thanks for your patience...

Bry

Ok. Here's where I'm at with this thing. I swapped the coils and took the dingy out for a hard run: about 35 minutes with two people in the stern and plowing lots of water. Engine sounded strong. After, removed the plugs... both had cleaned up nicely. Both had tan-colored insulators and were dry. Next, idle engine for 30 minutes. After, plugs were still clean but look a bit wet. Came into the dock under power... seemed to run well but the test will be tomorrow morning when the engine is cold. Let you know.

Thanx everyone,
Bry

Started it this morning and it's still missing!
Any thoughts....
Bry