What color is this 'fouled' spark plug? If it runs better with the choke out that in, it sounds like the engine may be running lean rather than overly rich.
Here's what I'd do:
Take the carburetor off again. Turn it upside down and take the float bowl off. Check your float and inlet needle VERY carefully. I suspect your float level is too low, causing the engine to starve for fuel and/or the floats to stick. Proper float height is 22mm (I think! Somebody correct me if I'm wrong!).
If the float is set properly and isn't binding, take the main jet out and look at it. I don't care what size jets were in the carburetor kit. What size is actually installed in there? (The main jet is the one which screws up into the center of the carburetor body once the float bowl is taken off.) Main jets are numbered, and the jet should be a 230 or bigger...
Also, be sure the carburetor cap is tight and the carb is firmly-clamped in the intake boot.
2-strokes are simple, and you'll get it figured out, but there is a little learning to be done...