well.. that's kinda weird .. but the excess smoke might be a clue..
Try this: Disconnect the oil injection system .. (be careful here .. don't just run the pump dry. The idea is to somehow route the pump's oil line so it pumps oil back into it's own tank.)
And plug the oil injection hole in the intake manifold (to avoid a vacuum leak through that hole).
Premix and run half a gallon of premixed gasoline:oil. Check the owner's manual for proper ratio of gas:oil.
If both the flat spot and the excess smoke disappear, you can probably blame them on the oil injection system. It could be just delivering oil erratically or pushing too much oil... and is either fouling the plug or upsetting the air:fuel mixture at some RPM range.. or the system may just need some normal maintenance.