I had a similar problem when I picked it up a year ago. Would only run above 4000 rpm. After trying several times I did (for some reason) open the airbox cover just after shutdown. The airbox was completely filled with smoke and gasses. Looked like a exhaust smoking at the wrong side of the engine. Refurbished the carb and the same problem with the gasses in the airbox. Ordered a china CDI for 25 euro and the bike started straight up and run like new. So let it run above 4000 rpm then shut-of the engine and open the airbox cover asap. Filled up with gasses and smoke, buy a cheap try out CDI to check. Have fun and drive save!