|Adventures Under the Hood|
The Decel Valve is behind the air cleaner, and is situated between the large manifold vacuum fitting on the back of the carburetor and the upstream Pulse Air tube. When you take your foot off the gas and decelerate, manifold vacuum soars, actuating this valve and opening it. This allows the manifold to draw fresh air in below the carburetor. It's a pollution control thing.
If this valve should leak or stick open, the result would be a massive air leak into the manifold, driving the computer crazy.