15hp 2 Stroke Johnson Outboard Issues - Spark Plug Fouling

[fishnNauru]

Gday From Nauru. I have an old 91’ Johnson that I’ve owned for a few years, bought second hand in Oz. This is my first outboard to own…and it’s been a steep learning curve at times
The motor has run well with a few issues from time to time. Since I’ve owned it the lower spark plug eventually fouls up with gunk and needs to be replaced.
I did a de-carbon on it recently and have since put a thermostat in it.
When I bought it, it had no thermostat so I have since installed one, thinking that it running hotter may help the problem, but does not seem to have.
The tell tale was abit slow the other day so I blew compressed air up to to clear it out and the water is flowing better, but the plug seems to foul quicker. Today it was running rough with gunk on the plug after being cleaned before heading out and it fouled and I replaced it and it fouled within seconds too.

The top plug is fine and does no foul at all, and I have no idea why the bottom plug does. I have not taken the motor apart to check piston and rings etc and don’t really want to, but it running rough is getting frustrating.
Has anyone had a similar problem before and does anyone have any idea what it might be?
Would blowing compressed air up the tell tale damage the water seals?
I run OMC oil in the fuel at the recommended 50:1.
Any tips or advice would be helpful.
Thanks FishnNauru

[TonyT1949]

Check lower cyl compresion. Probably broken piston ring. If comp. is good and within 5/10 lbs. of top then change the ign coil lesding to bottom cyl.. good luck