Not familiar with GMS engines but it sounds more like a fuel feed problem than carburetor adjustment to me. The problem it seems to me is in your fuel tank. Is your clunk free to move to all parts of the tank ? Are any of the fuel lines subject to pinching if the aircraft is moved beyond level ? If the clunk is rubbing on the back of the tank and cannot move it will not allow fuel to be drawn when the nose is elevated. If the nose is down same proposition in
reverse no fuel, lean run. Worth a try to check it.