At the top of the needle is a straight portion of varying diameters and lengths. This is what covers from just above 1/8 throttle to about half depending on the individual needle. The diameter is chosen to accommodate overall fuel demand and the length of the straight portion is chosen to cover demand at the higher end of the throttle opening,i.e. transition into the taper and main jet range.
The needle jet/emulsion tube determines the raw amount of fuel available across the range and the needle taper is used to fine tune delivery in individual portions of the range.
To answer your question the combination of slide cutaway (determines air available just over idle to just under 1/2),pilot/pilot air circuit,emulsion tube and needle taper tailor the fuel curve to the demand of a particular combination. You have to remember all circuits are additive meaning they all combine to determine the fuel delivery curve.
Sometimes the pipe,state of tune and carb combination leaves you with a flat spot that cannot easily be tuned away. It's either too rich or lean,the right amount cannot be delivered across the operating range. Sometimes if the advance curve is adjustable you can tune it away/minimize it if the fuel curve cannot deliver.
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