What the!!

What is that valve at back of my exhaust manifold???

 

It's the heat riser valve. When working properly it should be closed when cold, which directs the exhaust heat back through the intake manifold to heat the choke coil. As the engine warms up the valve should open and let the exhaust out the manifold on that side. Be sure to check if it's free and not stuck.

 

With out a picture I can only speculate, but it's likely there to cause back pressure to put more exhaust heat in the cross over in the intake manifold for the carburetor. Keeps fuel from puddling too much in the intake when the engine is cold.

 

Tree'd. I forgot about the choke. Yeah that too.

 

There seems to be a spring that is broken on it... would that cause it to remain open all the time or closed?

 

Damn predictive text!!!! Would that cause it to remain open or closed?

 

I believe there is a bi-metalic spring on it that when hot, will open the valve

They tend to rust and get stuck.
if your only driving your car in the summer or live in the south, you might have an exhaust shop remove and replace with a straight pipe

Kinda looks like this?

 

Yes, that's it.

 

Here's something to think about; A peice designed to delete the valve.
http://www.v8buick.com/showthread.php?t=181056