The Stovebolt Site Forums The Shop Area Engine Shop 235 manifold heat valve spring

I've experienced a problem with the heat-sensitive spring not opening enough. With the introduction of our winter-blend gasoline here in Colorado, vapor lock got me the other day at an outside temperature of 65 degrees! I installed a new spring and tested it with an electric heat gun with the manifold both off and on the engine. It worked fine then but in the real world not so much. I installed it with 1/2 turn of the spring to the anchor pin as illustrated in the shop manual. It seems that the engine doesn't generate enough heat to fully relax the spring to open the valve in the manifold with the engine running. Since I've completely unanchored the spring, my vapor lock symptoms (easily viewed by the percolating gas in the glass-bowl inline filter) have greatly diminished. The engine runs a little rough when cold but much better when warm. So, is there a happy medium to be found in modifying either the spring or the anchoring point? This is on the 48 suburban. My other trucks are also prone to the percolating gas routine but not as badly on summer gas. I live at 7600' elevation. Does gas boil easier at this altitude? Were these springs unreliable back in the day? Any thoughts welcome. Thanks!