Either buy an os 7d carb for your St or replace the whole thing with a new OS 160! The 160 is lighter and has loads more power. I had the ST 3000 and it was a good motor as far as reliability and easy starting, but the OS has the same attributes and will outpower the ST anyday. Just my thoughts. I sold my ST to be abl to get the OS and I think it was the best decision I have made.