Can't seem to reply directly to this thread, keep getting a message about posting HTML even though OE is set to plain text for NGs?
Anyway, if you don't know what is in the tank, the first thing to do is feed it some nice fresh 50:1 petroil mix. Then try screwing the air cap on the carb out a bit to weaken the mixture. It is unlikely that you will get it to
100% two-stroke with no load, but it should be tolerably smooth and a smoke screen and oil slick from the exhaust are by no means an inevitable part of S-T ownership!