Do you have a mechanical or electrical fuel pump? If it's the former, it's a simple and easy (read: inexpensive) fix. Rebuild the pump (I actually prefer the mechanical ones, as they are easier to deal with....)

The diaphragm in the mechanical fuel pump has developed a small leak, allowing fuel to enter the crankcase.

However, if you have an electric pump (and the fact that the plugs foul quickly seems to indicate this), you have over-fueling. One of the advantages of the Zenith updraft carb is that it is hard to flood it. However, if the pump is developing too much pressure, or the carb float isn't, or the needle or seat is bad or partly blocked, extra fuel can accumulate in the carb and be drawn off by the scavenge tube.

Extra fuel in the cylinders is a Very Bad Thing, as it washes lube oil off the rings and cylinder walls.

Good luck