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 J
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

[Fed]

Compression test, spark gap test, use specified brand plugs with correct gap and check the pulse side of your fuel pump for fuel leaking from a split pump diaphragm.

[Spaniard_King]

Use a champion QL77JC4 spark plug not an NGK

get some OMC/BRP engine tunner and decarbon the engine as per the can (even spray some down the plug holes after it stalls out on the tunner) also leave over night not just 15 mins

Check the mixture settings and make sure you use 95 octane fuel at 50:1

[tunaticer]

Check your fuel in your tank, sounds to me like it might have copped a bit of odd fuel (diesel or similar) by mistake sometime. Instant carbonisation sort of points to either excessive oils in the fuel or too cold a plug.
I run surface gap plugs in my old 2banger and it never has a problem with carbonising the plugs. L78V i think they are in my 25 rude and they would be nigh on 12 yrs old now i guess and still run fine.

[gazza2006au]

if your engine has only 1 carby the problem will be your coil or spark plug wire not producing enough spark making the bottom spark plug foul

[fishnNauru]

hey all,
I’ve been using the recommended champion plugs since I’ve owned it QL77JC4.
The engine has only one carb and same fuel obviously goes to both cylinders so I can’t figure why one fouls and one does not.
Spark seems good and equal when I pull on it with the plugs out and grounded to the block (after I clean the gunk out) swapping the plug wires has not made a difference.

Compression was good and equal in both cylinders when we tested it a while ago.

I did a sierra decarb just spraying the can in the carb then letting it sit and running it for a while after as the can recommended. I was wondering about doing it again spraying a bunch in each plug hole.

Anyone ever had similar issues in their outboard before?

Thanks for the suggestions.
FishnNauru