Sounds rich at the low speed and lean at the high. Your ST should have a long skinny needle valve (high throttle adj) and a short fat needle (low throttle). When you start it go to about 25% throttle and pinch the fuel line with you fingers or hemostats.
If the engine speeds up rpm when you pinch off the fuel line (it is rich at low speed), turn the slow speed adj needle in until when you pinch it you only get a minor speed up in rpm. Keep lowering your throttle as you do this too.
Then go the the high speed, turn it out about 2&1/2 to 3 turns. Set the throttle tx stick as high as you can and it keeps running, pinch and adjust the high speed needle. If you do not get a speed up, then turn your high speed out and repeat the process. Most likely you will end up with the HS needle about 2 turns or more out.
This works for most engines, pinch the line, engine speeds up a lot, it is quite rich. Engine speeds up a little and then wants to die, quickly let go of the pinch and it runs good, YOU got it!