This is purely my own opinion:
Long periods of wide open throttle @ 50:1
My next door neighbour knew better than my advise and went by the instructions on the Stihl 2-stroke oil bottle and had to walk home.
When he pushed the bike into my garage with a sorry look on his face, my response was:
Got his motor running again and he's now running a filth spewing, black tar like sludge oil ratio that leaks goo the same consistancy as a Transocean oil spill, from every corner of his engine, but his motor is still running.