This is your high speed adjustment screw. With the engine running full speed, slowly turn that screw counterclockwise very slowly until the engine picks up and runs smoother. If it gets worse then turn it clockwise until it smooths out. These are very small adjustments, so go slow. You will find a sweet spot where it seems to run best.
As a reference point the initial adjustment is 1 1/2 turns out from lightly seated (turned all the way in)