Could it be that you had fuel in the tank that had sat for awhile just before the plug fouled?
I've always felt that there is a chance of some two stroke oils falling out of suspension when sitting. Especially an oil with castor.
If you have a concentration of oil sitting just above the petcock, it is possible to introduce massive amounts of oil right away.
This may not be remotely the case in your instance, just something to think about.
Shake well before using. :)