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The captured needles have a dowel pin or stem that the fulcrum arm holds or "captures." This keeps the fulcrum arm in contact with the needles at all times. The captured needles are usually used on carburetors with the pump stack plates facing downward and mostly on 2-cycle engines.
Beware of aftermarket fuel inlet needles and seats. This is the number one problem after a carburetor has been recently rebuilt. The aftermarket kits usually do not seal very well. This is most noticeable at idle and low speed; the carburetor will be flooding fuel, especially from the high side circuit. Use a pop-off pressure gauge to check your pop-off pressure and make sure the needle and seat are not leaking.
Check your carburetor and carburetor listing for the correct inlet needle.