We have found number 21 series in 35 as to be a stock jet size in most GY6 carbs. By taking this up to 45 in our shop we have elminated problems with idle and midrange. Your pilot jet is used in half your throttle range at least with main kicking in at full throttle. Because the pilot is smaller it gets clogged even more easily than main.
When having gas flow problems in our shop we simply replace idle and main as well as air filter, gas lines, gas filter, fuel valve and by doing this we have reduced future problems plus upgraded the fuel and air flow which greatly increases performance.
Choose size of your pilot jet Stock is 35. A complete cylinder, exhaust and air upgrade go with 45. Exhaust only 40. Air 38-40. Need help call 301 351 7330