Did you tip it too much when draining fuel? With four strokes, the sump oil can go to places it shouldn't when you tilt it the wrong way, particularly tipped toward the intake side. It sounds like the vibration of the motor is getting trapped oil somewhere to sporadically drip into the combustion chamber.
Maybe find a location where the breeze carries any smoke away from neighbour's homes to test for longer and maybe burn away this errant oil.
