I have a ST 90 It ran well out of the box the idle settled down after about 6 tanks of fuel, the only issue I had was the muffler It fell
off the stub on its first flight lucky for me I managed to find it, I fixed this issue by drilling and tapping a 3mm bolt throught the stub and the muffler :- >no more problem

This link may be of some use

http://www.lymanslack.com/Articles&Archives/st.htm


Johnkpap