This is another of those scooter things that are so cheap to replace it's not worth getting too involved in the details. When the wires are connected to a battery, the brass needle-valve should extend about 3mm after 5 minutes or so. If it doesn't, then throw it away and buy a new one. They rarely fail, and the clearest sign of failure is your scoot won't start. The needle will never get stuck in the carb body, so if it doesn't come out of its socket, just give it a good yank.