The spring closes the heat valve in the exhaust manifold to direct exhaust heat right up under the carb for faster and more efficient cold starts. As the engine warms, the spring relaxes and backs off the valve. You still get some heat to the carb base with it not working, and in some warmer climates, you can get away with that all year long. If you are in colder climates, you should see about getting it fixed or replaced. Start by working the valve in the exhaust manifold. Use lots of solvent and slowly work it each day till it becomes free enough to rotate. Some of the venders in the LINKS above sell new springs.

Joe

1937 Chevrolet Inline-powered Streetrod