Jumping into this old coversation as I find myself having the same issue with my heat riser.
Installed a new Spring (Heat Riser Shaft moves easily) in the slit on the other end of the counter weight. The spring is snug on the shaft. I did look over the manuel and mentions to not over tighten the spring and only give it 1/2 a turn so the one end rests up against the manifold peg.
So when I installed the spring, I have the Counter weight in the HEAT ON Position, slide the spring where I can then apply some counter clockwise pressure to the spring to slide it past the peg. All looks exactly like it is pictured in the manuel, however when I take the truck out for a drive (3100 53 Chevy Truck with 235), the heat riser stays in the HEAT ON position. When I park it back in the garage to look at the engine, I do notice the Heat Riser jiggles a little like it wants to turn off, but never does..
