Just cleaned the carb (well, the top half of it) not long ago, the whole thing looked clean as a whistle. As for the airbox, I will go have a look...
...hrm, the airbox was popped open, not like all the way but it was not sealed... ganna go start it up and see what happens...
...seems to idle ok now, cant really run it for a stretch tho. Still ganna check that plug tonight.
I will keep an eye on that airbox though, guess if it was running on way too much air it might make it four stroke at every speed (until it gets enough gas in the chamber to fire).
Odd, when your thinking these problems through... it is always GCS (gas, compresson, or spark), I figured on it being spark, shows how little I still know :D