Erratic RPM's - Yam15hp

[a64pilot]

You did either replace with new plugs or at least swap the plugs, right?
If so then obviously one cylinder is running richer than the other, has a bad ignition system (probably coil) or maybe the crank seal is leaking, or maybe the compression is down on one cylinder.
Another possibility is the cylinder with the plug that looks good is actually running lean due to a vacuum leak and the rich looking plug is the good cylinder (unlikely, but possible I guess)

I'd swap coils to eliminate at least that, not much else you can simply swap to see if the fault goes with the swapped part.

[a64pilot]

This thing has independent coils? After thinking a little, it may have only one coil and use a "lost spark" ignition?
Wasted spark - Wikipedia, the free encyclopedia

I believe it's applicable to two strokes as well?

[impi]

You may well find that a piston ring got stuck ... that happens to 2 strokes when they do not have enough throttle ... they carbon up. With new plugs installed and running the engine at a 1:100 mixture at good throttle under load ... you may find it eases up ... just another thought for you before you start stripping too much

[sparrowhawk1]

If you replace the plugs and the bottom one fouls again. Take it out and ligh it against the side of the engine and see if you get a strong spark if you do most likely it's a stuck ring like mentioned earlier. if you don't then reinstall it and start the engine at night and look to see if there are any sparks jumping anywhere

[bcripps]

Thanks 64pilot: Yes, I cleaned the bottom plug and swapped it to the upper cylinder which seems to be running fine. I did this a couple of days ago and have not checked the condition of the plugs since. Wanted to give it a few days. Engine, as of this morning, is still missing.

You mentioned swapping the coils. Each plug wire goes to a black cylindrical part mounted on top of the block. TWO Coils? Can I just pull out the plug wires and cross them over?

Thank you impi: I have been running it harder the last few days and using the RedLine fuel system cleaner that is reported to help break up carbon, but who knows. I'll give it a few more days.

sparrowhawk1: You're suggesting that I might have dirty wires, causing a short? That's certainly possible. And running at night sounds like a good way to find out. Thanks. I'll give it a try and get back to you!

Thank you all...
Bry

[a64pilot]

If the plug wires go to two different components, then they can be switched, not by just moving the high tension wires though, you need to move the whole component over.
The intent is to fault isolate the problem to a particular component.
Sometimes one coil is used, with two high tension leads, what happens then is the plugs are fired when one is at TDC, and the other is at BDC, that is the wasted spark ignition.
A picture of you coil or coils would help.

[bcripps]

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 hazzarding a guess here, that both plugs are functioning and, as others have suggested, it might be a coil or a sticky piston ring. While I had the cover off, I removed the spark plug cap on the lower cylinder. No evedence of an electrical arc but the spring-like clip is not exactly a snug fit. It just 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!

The photo here, is of the coils. The plug wires seem to be permantly attached, but haven't fiddled with them yet. It's raining. One plug wire is much shorter than the other. Not sure I can swap the coils and wires and reach the lower cylinder with the shorter wire; but I'll give it a try!

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

[impi]

Originally Posted by bcripps

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 hazzarding a guess here, that both plugs are functioning and, as others have suggested, it might be a coil or a sticky piston ring. While I had the cover off, I removed the spark plug cap on the lower cylinder. No evedence of an electrical arc but the spring-like clip is not exactly a snug fit. It just 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!

The photo here, is of the coils. The plug wires seem to be permantly attached, but haven't fiddled with them yet. It's raining. One plug wire is much shorter than the other. Not sure I can swap the coils and wires and reach the lower cylinder with the shorter wire; but I'll give it a try!

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

Bry ... I would really take a look at that cable ... the cap should fit snugly onto the end of the plug and you should get a good spark from it if you keep the plug grounded with the wire attached.

It may not be a bad buy to replace that wire complete with cap ... can always keep one spare!

Have you checked spark on the other plug compared to the lower plug?

[bcripps]

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 looked 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

[a64pilot]

Sounds like a loose coil wire as in spark plug boot end.
Used to be and I bet you still can replace just the boot and the spring like wire in it, for like $3 or $4 bucks.
Called I swear if I remember right a "sparky" spark plug boot. The kind you see on old motorcycles, chain saws etc.

[a64pilot]

Look for something like this

https://www.brillman.com/store/produ...ition-terminal

[bcripps]

Started it up this morning... still missing!
Further thoughts are appreciated...
Bry

[impi]

Originally Posted by bcripps

Started it up this morning... still missing!
Further thoughts are appreciated...
Bry

Just to re-cap since this is a 'cold issue' now ... your fuel mix is 100:1 right?

50:1 is more required for newer engines prefer 100:1.

And your fuel is good quality for 2 stroke - 2 stokes do not like fuels with high ethonol content ... I think it should be less than 10% if memory serves me correctly. I remember in the Bahamas we had to use Fuel stabilising agents. The engine struggled and spluttered if idled too long. In any case, the 2 strokes do not want to idle too long as they foul up ... they like revs.