Doesn't matter- - - - -all that head is good for is an anchor for a medium-sized boat! The way it's supposed to work is the flap valve will be in the "hot" position pulling against a spring when vacuum is applied, and a thermal valve cuts the vacuum off when the thermostat opens and vents the line when the engine reaches operating temperature. The default position is "cold" to prevent detonation if there's a vacuum loss for some reason. That vacuum valve will not function properly without at least a 195 degree thermostat. The whole mess was a Rube Goldberg method of trying to meet EPA exhaust emission standards during warm-up. Installing an earlier cylinder head/manifold assembly with a regular thermostatic coil heat riser would be a much better option.
Jerry
X2
integral head / manifold 250 tend to like to crack, & the Vari-jet carb is a pain (IMHO),
