The heat riser valve is used to redirect some hot exhaust gases back into the intake manifold to warm the engine up quicker. When the valve is in the horizontal (closed) position, it forces exhaust gases back through the intake manifold into the engine to warm it up quicker. (All of this happens at idle. At higher RPMs, the valve is forced open by the speed of the exhaust flow.
If the valve sticks in the closed position, the engine will not run well at all when it's warmed up. If it's stuck in the open position, it will take longer to warm up and may affect performance for the first few minutes the engine is running.
To answer your questions:
When the spring is missing, the valve will remain in the vertical position (wide open, because the spring's purpose is to maintain tension to hold the valve closed until the engine is warm.
The symptoms, when it's not operating will either be excessively long warmup times (if it's open) or poor performance and detonation when the engine is warmed up (if it's closed.)
You should notice a slot in the end of the rod that the valve pivots on. The spring fits in there, and the other end sits against a small rod-like projection on the side of the manifold. The spring is installed so that, when tense, it holds the valve closed.
Yes, the spring is a bi-metallic spring that expands unevenly when heated.
