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Old 07-30-2009, 04:49 AM
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Vacuum Gauge

I recently installed a NOS dash mounted vacuum gauge in my '64 Bonneville. But the readings seem to be wrong. I get 11 hg at idle, so the needle is in the middle of the scale (marked orange). As soon as Iīm driving, I just get 8 hg (marked red).
Even when rolling downhill with the foot off the gas pedal the needle wonīt reach the green side of the gauge.

I connected the vacuum line to the left port of my qjet (when standing in front of the engine). I used a gas-hose with 0.1'' inner diameter.

Could this be the reason for my false readings?

thx

Markus

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Old 07-30-2009, 08:02 AM
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It needs to be connected to manifold vacuum. If memory serves, the line should be 3/16". Find someone with a mitevac hand pump to check accuracy of the gauge. They were notoriusly inaccurate, but were eyecandy anyway. I have several and would put good money on none of them reading the same.The factory tach guys love them cause the tach fits the same housing and vac housings usually aren't scratched up so bad.
Incidently, a warm cam will throw the reading way off.
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Old 07-30-2009, 09:32 AM
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Thanks a lot 61bone!

Does anybody know how much vacuum I should have at normal cruising conditions? Itīs a stock 301 pontiac V8.
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Old 07-30-2009, 10:17 AM
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Just normal cruising, holding a steady throttle, and your engine being stock, it should show between 17-19. Also, right after you take your foot off the pedal it should jump above 19, and into the low 20's.

Retarded ignition timing will lower your intake manifold vacuum, and I would suspect the vacuum advance on your distributor is bad, or it is hooked to a ported vacuum source.

Make sure you have it hooked to a manifold vacuum source, and unplug it listening for the engine idle speed to drop. If not it is bad, and causing a vacuum leak.
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