Couple of notes for you,
Generally if the threads are wet, it is oil working its way up the threads. If the engine is new, i wouldnt be too worried its still breaking in and you will see some oil now and then till everything is seated.
You adjusted the Idle air screws, not the air fuel ratio, as soon as you pick up the throttle those adjustments are moot.
If you have a "surge" just as you come to a stop, either the idle is too low, the convertor is too high of a stall, possibly a high/low float, rear idle circuit is too high (dropping fuel with the blades closed) or even a broken advance weight spring. Hard to say.
If you want to start eliminating the carb, srcew the rear bleeds all the way in and cut the flow. If that helps somone has probably bob vila'd the carb and turned the rear blade stop in or out under the carb feeling they were doing something trick.
As far as the plugs go, the best advice was to swap the black plug to a good hole.
If it persists start looking at things that would make a cyl run rich, weak spark (bad wire, contact on cap, star wheel on the shaft damaged) exhaust gas entering the chamber on intake stroke, tight valve, oil entering into the chamber like bad valve seal, bad ring, bad guide etc.
hope you figure it out and have fun.