I saw one guy out at the field screwing around with a ST with similiar problems.

Since others were helping him, I didn't think to step in.

However after I saw him fighting with the engine for DAYS, I finally stepped in.

He had the "cat eye" facing UP instead of down.

I remove the needles, flipped the cateye over, stuck the carb back in and the problem was gone much to his amazement.

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Also pull out the carb, put a CLEAN piece of fuel tubing on it, and with the the throttle wide open, blow through the tubing.

You should hear and feel an audible escape of air.

Close the throttle while doing this and air should continue to flow to a lesser degree until the barrel is completely closed.

If all else fails take the carb apart, and clean it out. I've found debris in ST carbs that affects transitions and idles.