ORIGINAL: Harry Lagman
Dar, I doubt that the throttle setting Cyclic is talking about (the setting that is less than full throttle where rpm is at, or close to peak) is greatly influenced by the low speed needle.
As for the unloading factor, that is a valid observation and as you say could affect performance where maximum speed is desired. The application I am tuning this 2300 for is a Funtana 90. The owner only uses full throttle on his vertical uplines so there should be very little unloading in normal flight.
Harry,
In all two-needle carburettor, the taper of the idle needle, and/or the shape of the fuel-jet metering slot, meter the amount of fuel the engine receives from 0% (idle) to 75-80% of full throttle.
Before Cyclic edited his opening post, the question was about this particular engine model, reaching maximum RPM, with the throttle less than fully open; 2/3-3/4, I believe. This is why I expressed myself as I did.
Since at these partial throttle settings, the fuel control is done by the low-speed circuit, even if the main needle would be opened several complete turns, it would change the amount of fuel that the engine receives, by very little.
The low-speed 'circuit' is the bottle-neck.
...But closing the main needle further would reduce fuel flow, since at a certain point it becomes the bottle-neck.