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Jul 21st, 2025 | | | | Joined: Oct 2007 Posts: 397 Shop Shark | | Shop Shark Joined: Oct 2007 Posts: 397 | First question, when running at idle is the vacuum line at the dizzy suppose to be zero? I disconnected the vacuum line at the dizzy and hooked up my vacuum gauge and and at 475 RPM idle it reads zero.
When I reconnect that vacuum line what would be a good reading range as I increase the speed of the engine. I'll probably have to put a "T" connection in the line in order to get a any readings.
I can't seem to find any written ratings in my shop manual for this vacuum line.
"Lucille" ..... Proud Member of the "Southern Stovebolts" David Wolff 1946 Chevy 1/2-ton In the Gallery | | | | | Joined: Oct 2007 Posts: 397 Shop Shark | | Shop Shark Joined: Oct 2007 Posts: 397 | I just checked the vacuum at the side of the intake manifold and at idle its reading 18 inches. The other line at the dizzy coming from the carb is reading zero. Once again is that normal? Trying to figure out this power falling off problem.
"Lucille" ..... Proud Member of the "Southern Stovebolts" David Wolff 1946 Chevy 1/2-ton In the Gallery | | | | | Joined: Dec 2008 Posts: 1,915 'Bolter | | 'Bolter Joined: Dec 2008 Posts: 1,915 | You're tapped into "ported vacuum", where the throttle disc is below the port at idle. As the throttle opens, the disc sweeps past the hole and vacuum reaches the distributor.
You'll get at least 2 very strong opinions as to which is "correct": ported or full-time. Based on prior posts, those who are sure of themselves and insistent have the least grasp of how it works.
The answer is: use whichever one works. | | | | | Joined: Feb 2004 Posts: 28,674 Kettle Custodian (pot stirrer) | | Kettle Custodian (pot stirrer) Joined: Feb 2004 Posts: 28,674 | The engine was designed to run ported vacuum to the distributor, which will only be there at part-throttle cruise. Then it will be somewhere in the 15-18 inch range. As soon as any load goes on the engine the vacuum goes away and the distributor returns to initial + centrifugal advance. No ported vacuum at idle, and slightly less than full manifold vacuum at cruise. Setting the timing to the BB with full vacuum at idle will result in very severely retarded ignition timing and very low power.
Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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| | | | | Joined: Jun 2004 Posts: 8,597 Riding in the Passing Lane | | Riding in the Passing Lane Joined: Jun 2004 Posts: 8,597 | The old Stovebolts used ported vacuum for the dist.It will only have vacuum at part throttle. You can watch the dist vacuum work on this by watching the dist as you open the throttle. You should see the dist turn at part throttle. Vacuum advance is basically for economy. It will not effect "power falling off".
They say money can't buy happiness. It can buy old Chevy trucks though. Same thing. 1972 Chevy c10 Cheyenne SuperIn the Gallery Forum | | |
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