I know this is a stretch for "All Other Vehicles" but maybe someone has some 2 stroke outboard knowledge since this is a very diverse group.

Got a '78 15hp Evinrude 2 stroke engine, and I keep losing the upper cylinder. The wire when seated on the spark plug won't complete the electrical circuit. If I remove the wire, and hold the boot just onto the edge of the plug, the circuit completes and the cylinder runs normally.

Why would a spark plug not fire when the wire is seated on the plug, but if it is off by about a half inch to an inch, the cylinder runs normally?

The plugs look fine. I swapped the plugs from the other cylinder with the same result.

I've had all of the electrical components replaced recently because of this very problem. It ran fine for about 15 hours total before the issue came up again today.

Thanks in advance if anyone can help.

I don't see an answer so I will guess that it's the voltage.

Take this example.
Assume a plug that is fouled for whatever reason.
That fouling can be caused by the lack of a high voltage spark.
Why the lack of a high voltage spark? Lots of reasons.
Let's just say the edges are rounded (for now) but lots of reasons can get you a low-voltage spark (meaning there is a trickle of leakage or a lack of concentration of the spark).

What happens when you pull the boot away?

What happens is that only the highest voltage spark zaps across, and, better yet, it happens all at once.

If you follow my theory through, what I am guessing can be happening is that you have a slight leakage where as the voltage winds up (nothing is instantaneous) some of it leaks down the tunnel.

But if you pull it away, nothing leaks until the full jolt is felt.

Just a guess though.

Thanks for the response... we found the issue.

So: the '76-'80 8/9.9/15hp Evinrude/Johnson outboards were all pretty much the same during this time range, except for the fact that in '78, they went from points to a power pack. At some point, the pack must have failed, and someone replaced it with points (prob bc they had them laying around?). Everything electrical downstream of that is slightly different than the setup with the power pack, so as we replaced parts, we were doing so assuming it had a power pack, thus actually making things worse. We left the points since they were working fine, and I still kept some of the other parts I had initially swapped out, so in the end, the fix only cost about $100. Running perfectly since.

Did that make any sense?