First off take a good look ant your fuel tank and lines, including the stopper and clunk line in the tank. Even the tiniest air leak can cause issues.

Once you are confident all of that is OK, make sure your carburetor, back plate, cylinder head, etc. is tight and leak free.

Fire the engine up and let it get warm. Advance to full throttle and adjust the high speed needle a small amount at a time until you achieve maximum RPM. You should know this just from the sound; it will be screaming. Then back the high speed needle out a few clicks until there is a slight RPM drop. It is always better to be too rich than too lean on this setting.

Once the high speed needle is set, it is time for the low speed. Pinch the fuel line going to the carb. If the engine quits almost immediately it is lean. If the engine speeds up noticeably and runs for a while then it is rich. If it runs for a short time then fades & quits then it is likely close. Adjust as needed. Verify your adjustment by checking the transition. It should be smooth & quick. If it immediately falls on its face & quits then the low end is too lean. If it hesitates & spits a little then it is likely rich. Adjust as needed to get a good transition and reliable idle.

Once the low end is set, go back and check the high speed again and adjust if necessary.

Another thing to consider is your glow plug. Whenever you are having engine trouble, put in a new one. This will at least rule that out or it might even cure the problem.

Another tip that I think a lot of people need to learn is that once you get the engine running good, don't mess with it. I see too many people who ruin their flying day fiddling with the needles. It shouldn't need any other adjustments unless you change fuel types or there is a significant change in the weather. Even then, it should only need slight adjustment.