I have a 2004 15hp 2 stroke Yamaha. Bought it about 5 years ago and never had any issue. A couple of weeks ago I took it out on my inflatable. I was wot for a mile or so and went to an idle and shut it off. When I restarted it, it wasn't running right, down a little on power. When I got home I pulled the plugs and found both plugs fouled (never happened before).
Last week I changed the plugs and tried to use it. As soon as it tried to go 1/3-1/2 throttle I knew it was down on power. Put 4 new plugs in and would foul right away.
I thought it might have been the gas i mixed (100:1 with yamalube) but haven't been able to try it with fresh gas because of a torn seam on the inflatable.
The other day i did a compression test. Both cylinders were exactly 70psi. Does that seem low? It's 6.8:1 compression.
I pulled the head to look at the cylinders. Nothing jumped out to me but I'm not a 2 stroke guy.

Yep low, should be over 100 and looks like you found some scoring, but I'm no expert

+1 on the scoring, in my jet ski days minimum compression was around 120, 70psi will def foul your plugs, another question I have is the 100:1 mix? We always ran 40:1, maybe 50:1, that sounds like it could be the problem right there, but then you said you've ran it for the last 5 years tho

That's not the answer I was hoping for, but was expecting.

Manual states 100:1. The Envirude engine this replaced was also 100:1. I wonder if I should be running it 50 or 75:1? Issue happened while running it hard. It has been wot quite a few times since I owned it with no issues up till now.

I have to look into rebuilding this engine. Was a fresh water engine when I bought it and always used salt away after each use since I owned it. It's a very clean engine. Seems the parts are pretty cheap, They sell stock and 2 different oversized pistons, .25mm and..5mm. Does anyone know how far these cylinders can typically be bored/honed?

I've always used the rule of thumb min 100psi compression for 2 stroke engines.

First time ever i read about 100:1 ! That is more than twice leaner than what i ever saw a recommended mix!

I've seen a full 1 to 2mm increase in bore size, there is more than enough material there to clean up the scoring, I would not continue the 100:1 ratio, minimum I'd do is 50:1, I'd also switch to amsoil over the yamalube, I've personally never been a fan of it

Those Score marks aren’t that bad for an two stroke, I would try Try a leak down at TDC before you start pulling pistons, better yet try running some ring free and check ur ignition it’s probably been low comps for a while and ur spark has gotten weak

what did the plugs look like, wet with fuel, black soot, ash, heavy oil?

Have used 100:1 synthetic for years in chain saws and weed wackers

Even the 50:1 oil injection dino oil motors runs from 100:1 at idle and go up as rpms increase