The heat riser allows exhaust heat to flow to the bottom of the intake manifold, increasing intake air temperature in the manifold, thereby increasing fuel vaporization and preventing fuel puddling below the carburetor. Blocking the heat riser closed can result in rough and difficult operation due to fuel not reaching the cylinder, especially in circumstances of low intake air velocity at low engine RPM.
If the heat riser valve was stuck open, exhaust heat will continue to heat the intake manifold, resulting in high intake air temperature, increasing the possibility of preignition or detonation.
I have a 216 intake and exhaust manifold assembly off the engine, I can check the heat riser operation later today and report back.
