Have changed the fuel lines, checked the o-ring. You can put a piece of fuel tubing over the needle valve to seal it. What is the tank height? Put your finger over the exhaust exit and see if it will go to the carb. Is the o ring under the carb good. Sounds like an air leak some where.

You will find threads about ST engine in the engine forum. I have had very little trouble from my ST engines. I have one GS-45 that has out lasted (2) OS 46's.

Dru.

Edited for spelling.