If it is a ringed ST, NEVER run it lean on the break-in, and only gradually lean the needle for full RPM over many short runs. Excess heat will destroy the ring/cylinder seal very quickly on a new engine. For the first few runs, it should be run with WOT, plug heat on and so rich that raw fuel is spitting out the exhaust. It should not be able to keep running without plug heating. You are trying to knock off all the machining dingleberries and rub in the ring and big end bearing, and anything nasty should be washed out with the exhaust. Rich also means that there is plenty of oil for lubrication and heat extraction. The engine will then thank your care and continued consideration with a long and happy life.
Evan, WB #12.