I've been thinking on your problem this morning during my travels!
Ok, you saturated the air filter with a "lot" of oil! To the point that the air filters excess oil could just drip off, right? So you cleaned out the air filter, but the stumbling was still there, right? Now we've all been wondering how "could" this previous saturation of oil it affect the operation of the carb. to cause a stumble, right?
I kept going over and over all of this in my head, and as to what you did to correct it and what still resulted! I just couldn't figure out or see "how" that could affect anything with the operations of your carb!
Then it hit me like a ton of bricks!...DUH!...Like I said :loony:
"It shouldn't nor couldn't"! and
"It Didn't"! 
The excess oil got "sucked" into the carb., right?
Check the "SPARK PLUGS"!......I'm :loony:
I have the feeling the oil saturation may have been great enough to completely make it through the carb., and straight into the combustion chamber and foul the plugs.
Im sorry! I may be slow"er" :loony: these days, but I'm still here trying! :lol:
"I think":lol: we may have the explanation here!
Good Luck!:2thumbs: