EDIT: I re-read your post carefully this time
Well... assuming everything is stock, clean and never touched (rejetted) before, and assuming it's the air / fuel adjustment that is the problem and everything else (fuel level in float bowl, dirty air filter, ignition, valves) is OK - turning clockwise (leaning mixture) in your case, on a warm idling engine should cause a slight
increase in rpm.
Let's see... First get the engine warm, no choke, idling. Turn the adjusting screw clockwise all the way. Engine should die. It should be about 1.5 turns in, factory setting. Jot down how many turns you had to make. Then turn back out 1.5 turns (360 + 180 degrees), and start from there. Crank up, test drive for few days, note any obvious symtoms (popping, hesitation, black powder
)... Then let us know.