I have used fuel engines since 1975, and I learned the only way to set the needles early in my "career". This have worked on all my fuel engines.
Despite the carburator is clean and everything else OK (fuel line, plug, fuel etc) I do it this way:

- Adjust full needle to max RPM and back off two clicks.
- Let the engine idle for 5 seconds, open the throttle abrupt and listen. If the responce is perfect I increase idletime to 10, then 30 seconds and check if it responds perfect.
- If it don`t the engine die/raise RPM in two ways: Either it stops/spin up sounding like it is leaning out. Then I richen the low needle (very small adjustments every time!), and readjust the full speed needle. Redo the idle test, and do further adjustments.
- If the engine die/raise RPM, making a noice like it is rich I do lean the low end needle, and do adjust the full speed needle and redo everything.

I have no idea how many times I have done this at the field helping beginners make their engines run reliable... Works every time!

But if the plug is wrong or the fuel is wrong or very old it is impossible making an engine run perfect. And some cheap china-motors is impossible to get reliable, usually due to poor carburators.

When adjusted my engines could idle for 2-3-4-5 minutes and still respond perfect every time