i'm not sure about the power valve's function, but i think you're going to have to take your carb apart. If it is draining dry then the first thing i would check is your main inlet valve. Check your floats and make sure that the valve is shutting properly. Also check out your accelerator pump and make sure that the rubber washer is in good condition and sealing inside its well. These are two main things that could be causing you problems. If your carb is generally in poor condition then i would suggest that you rebuild it.