You don't mention your brand of oil. Some oils such as synthetic 2-stroke oils work extremely well in so far as not fouling plugs. There are other oils that are quite poor by comparison. Are you using 2-stroke oil?
If it is running clean, i.e. not four-stroking then carburetion (jetting) is not likely to be your problem. That motor should not foul plugs unless something is very wrong, such as an extrememly weak spark, possibly due to timing being way off or ignition components or wiring being defective.
Another possibility is that it could be sucking transmission oil into the crannkcase through the right side crank seal. This could cause plug fouling and would also cause the transmission oil to become depleted. Check your oil level in the tranny.