Originally Posted by turk1
The mysterious 3rd needle is to adjust spray bar length(posiiton of spraybar mouth )So no need to play unless feel unproper placing of spraybar mouth.
Now I am just wildly guessing: the airspeed in a venturi is not uniform, but higher in the middle and slower closer to the walls. Higher velocity means higher fuel draw (the fuel pump/regulator just keeps the pressure at before the needles constant, the fuel is still drawn in by pressure differences in the venturi, as in any carburettor). Maybe that principle is used to influence the linearity and transition of the carburettor a bit. If that is true, it would make sense that a centered spraybar richens midrange, and a withdrawn spraybar leans it out. I remember on some old Enya's the spraybar could also be screwed in or out (although that was never the designers intention I guess) and turning that in or out could improve transition a bit.