[robn]

Our Yamaha 2 stroke 8HP (2004, I believe) has been fouling plugs since we bought it. It is now getting worse. Engine runs pretty good on new plugs then starts to get slightly rough and loses power gradually till the dinghy won't plane anymore. Sometimes running on only one cylinder. After about 3 gallons of gas the plugs need replacing. We have had it in to 4 different shops who all claim the compression is good and the ignition is good.
We have replaced the gas and oil (100 to 1 mix), carburetor parts, reed valve, thermostat, seals, adjusted everything, cleaned the exhaust system and the water drain, tried hotter plugs (#6 instead of #7), all to no avail. She is okay with new plugs but they don't last long. They get black and sooty/oily.
What's next?

[matt2002]

That's a good question.I run my 9.9 yamaha on the same fuel as my 75 , a 50:1 mix , and have no plug troubles at all.the 9.9 is supposed to run on 100:1.