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Apr 26th, 2026 | | AD Chevy Trucks  Over 6,000 pictures Brad Allen has an awesome collection of Chevrolet factory pictures that he has set up from film strips. Check them out!
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| | | | Joined: Jul 2007 Posts: 461 Shop Shark | | Shop Shark Joined: Jul 2007 Posts: 461 | A rule of thumb for manifold vacuum is a loss of 1" Hg per thousand feet above sea level. With that, you could expect 3.5" Hg less than the 17-21 level given. Even with the expected loss from altitude, your level of 15.5" Hg seems low. Make sure your initial timing is set correctly, and the idle speed is 550-600 RPM after setting the timing. The idea that your gauge is off is equally valid, but there isn't a good way to check that without a second gauge or a calibration set-up.
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