Long shot; but what oil ratio are you using? Rotary valve engines seem to like a strong oil mix, in my limited experience with them. I would think it helps the sealing. I think I ran mine at 16:1.
Good spark and compression would be my next suspects, followed by how the throttle picks up with relation to the mag position….I don’t remember much about how Champion did that. I do know mine ran incredibly WELL at idle; so you should expect a lot from yours.
Long live American manufacturing!