Sounds like tank problem. Sometimes the clunk gets stuck in the front of the tank in a "U" turn. When the tank is full, it will run fine even when pointed straight up. If you can get to the tank easily, do a visual inspection. If not, fill the tank only about 1/3 full. Start the engine and hold the plane in various attitudes (nose up, horizontal left side, horizontal right side, horizontal upside down. The engine should run the same in all positions. The only time it should quit is when you point it staight down. Also, as you are doing this, watch for bubbles in the feul line going to the carb.

Most glow engines, once started, will run the same in all attitudes.. Even if the needle valve is too rich or too lean, it will not change the engine by being upside down. That is why I suspect a tank problem.