The function of the heat riser valve is to speed up the warm-up process of the air-fuel mixture for proper atomization. When stuck open it will slow this process, and can also contribute to carb icing if not performing it's designed function during operation. Stuck closed, it can generate too much heat, and you will over temp the carb which will contribute to vapor lock. Generally it's an all or nothing proposition. It will not usually stick half way unless there is some sort of mechanical interference, say from an oil filter line.
