There is a way to test for air leaks while the engine is running. You need a can of WD-40. This was told to me by a mechanic friend that uses it a lot to fix other peoples problems. With the engine running at max throttle, spray around the base of the carb (not in the carb), around the carb barrel, remote needle valve, back plate, and head. If the engine sag's, that is a leak. Many times this doesnt produce any results. That at least lets me concentrate on other areas. I have also fixed a wayward carb by removing the needle valve and spraying WD-40 into the spray bar, or removing the fuel line and spraying into the spray bar to clean out any foriegn matter. Take a look at the glow plug for any anomolies like a non uniform coil. This could indicate an overheated condition. Also, check that the pressure tap on the muffler is clear and breaths ok. The engine needs some pressure in the tank to help fuel draw. 10% should be all that you need. Additional info like prop (brand and size), glow plug type would be helpful. By your symptoms, its sounds like a carb adjustment problem. But you've already done that dance, so the other things are the next step. After that, may need to send it in.
Edwin