I have an 05 Grizzly 80 doing the same thing.. Put a new plug in and it is fine for a few hours then acts like it's fouled out. Pull the plug and its pitch black and sometimes wet. I'm guessing maybe the carb needs adjustment but I have no idea how.. Anyone have a fix for this????
having just purchased a used 02 yfm80 i thought it ominous that it came with 3 used black plugs under the seat in boxes. it looks ok and sounds ok but wasn't what i'd call overpowered for an 80 4 stroke. after actually reading the manual i discovered that it has a plastic sheet with 6 holes in it in the intake box that is user removable for more revs. also it came with the throttle stop adjuster set to only allow around 80%throttle. i have since removed the restrictor and got full throttle movement and it now wants to pull me in 2nd gear uphill, it wouldn't before. anyway my guess is that in giving it as much throttle as you can IF there are any restrictions like the 02 model you will actually be running it rich at higher throttle settings as the airflow is restricted but the needle in the carb is set to let in a lot more fuel than the engine can get air to burn cleanly. mine needs a new battery so i haven't got even an hour on it yet but i am confident it won't soot up the same from now on.
Okay. Is there lots of black smoke when it runs? Try putting in a new plug, pulling the sparky out of the exhaust, and running it, and see of the plugs are still black. Sounds like you guys are running really rich. Which can be adjusted by changing the jetting the carb.