Thanks for the info! I'm assuming, then, that the flapper valve should be in a closed, or semi closed, position when cold and then when the manifold gets hot, the flat coil spring(s) will allow the flapper to go fully open. Does that sound correct? Reason I'm asking is that the springs and flapper are not in great shape. The entire manifold is very pitted. That is also the reason that I'm considering a header. I guess if I can't get the flapper valve working, I can just remove it.
