Chevrolet engineers spelled it out pretty good in the shop manual as to why it’s there and what it does:
“As the engine warms up the thermostatic spring closes the valve and directs exhaust gases away from the manual as shown in figure 3. This thermostatic control results in the proper temperature of the incoming gases under all operating conditions. The tension of the thermostatic spring is very important. When too tight, the heat will not be turned off of the intake heat riser as the engine warms up, with the result that the incoming gases will be expanded several times greater than normal and it will become impossible to get a normal fuel charge into the cylinders. This condition will reduce power and maximum speed and will cause detonation as well as sticking valves.”
http://chevy.oldcarmanualproject.com/shop/1948_51truck/51ctsm0604.htmFind another manifold and make sure it functions as designed for best results.
