Well... I found an OS 21 RX-R engine at OS's site in the discontinued page:
http://www.osengines.com/parts/xr23818000.html
No picture or anything.
It says the carb is a 20A which is also discontinued, but some of the parts appear to be interchangeable with the 20B according to tower:
http://www2.towerhobbies.com/cgi-bin/wti0097p?MANUFACTURER=OSM&FVSEARCH=20A
Parts like the HSN, LSN, HSN body...
So, I checked OS's site again, found the manual for the 20A and 20B (same manual with info about both carbs, slightly different)
Manual
After reading the manual... it says the mixture needle (LSN) is set from factory... the "Needle Valve" (HSN) should be 2 1/2 turns (20A carburettor)
or 3 turns (20B carburettor) from the fully closed position.
Further reading tells you that if you totally "F" things up, that the LSN should be 1 turn in from flush (20B) and 3 turns from flush (20A).
Quote from manual:
"Its basic position can be found by first rotating the
Mixture Control Valve until its .slotted head is flush
with the carburettor body. The valve is then screwed
in exactly 1 turn (20B carburettor), or 3 turns (20A
carburettor), to establish its neutral position."
Is it just me, or is
carburetor spelled funny? They have
carburettor.
I hope this is your carb/engine... or I just totally wasted 20 minutes of my life.