The needle position is not a restriction, something that you can change to prevent it from running lean or rich, just like the jet size. The needle diameter affects the mixture at 1/7th throttle to 2/7th throttle. The needle position affects 5/14th throttle to 6/14th throttle and the conic form of the needle affects ½ throttle - 5/7th throttle. Then from 5/7th throttle to 12/14th throttle theres a hole where the needle matters less and less and the jet size matters more and more.
0 to 2/14th throttle is the mixture screw and idle jet and 12/14th to full throttle is the jet size.
Unfortunately the needle position is not something you can change on a Kymco Agility, but you might be able to buy diffrent needles, where the diameter and conic form is diffrent, and where the position can be adjusted.
About the screw at the spring:
This was ealier used to prevent the needle to hit highest position, and caused the engine to be running lean on full throttle, as the needle would still cover some of the jet. This kind of restriction gave starting problems, and caused many engines to be ruined. It was only used on cheap chinese carburators and was never used on any Kymco engines.
Hope it helps, and the picture and information is taken from this site, which is danish:
http://scootergrisen.dk/scooterhjemmeside/karburator.php
