The low idle adjustment on an os (and most engines) are pre-set....They are set right most of the time.......(altitude and temp are varibles).....Most of us never mess with it after we establish a good run after break-in.
Rule of thumb.....don't mess with low idle screw unless you know what you are doing......
It is Ying and Yang with the top end rpm, and mid range.
Move one....requires a tweak on the other......
Go back to square one as per instructions provided....Start all over...
Sounds like your engine is fat with too much fuel.
Doc