Mc,
You might snoop around abit and start from scratch on the jetting,
(find out what the bike came with stock)
Getting everything cleaned up (sounds like you've already started)
Carb innards,(jets & passages) air filter and so-on.
While you have the carb off take a peek at the reeds, chips,cracks, gaps around body and petals etc.
I know you already changed the tranny fluid, next time actually measure how much you put in, and measure how much you get out after the ride.
I know it may sound odd, however if you have a crank seal going out on the tranny side, the engine will actually draw trans fluid in the crankcase.
The last time this happened to me I could actually smell the tranny fluid on the plug...
You may even be on to something as far as just a bad plug,
It may be rare but not unheard of.
Tuck\o/