You may want to read up on the dual diaphram setup. The only time the vacumme diaphram advances is when it gets really hot, if I remeber right. I don't recall what the Thermostatic switch switches at but its over 200. Pontiac put out a service bullitin about it because of overheating issues. The 69 cars ended up having the thermostatic switch but not the dual diaphram after that. I researched it a bit when I was looking at mine. Someone hear should have the info.