oops....yes....skybolt58 is right. Maybe it is just the sample answer. Thank you skybolt58.

Pttf,please close the throttle more with the idle screw(counterclockwise). Till the engine almost flame out then screw it a little.

Here is the new 21/28 manual. Maybe someone need it.
http://www.stsrc.com.tw/info/21%2028...ual%20v3.1.pdf