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Author Topic: Vacuum Advance  (Read 845 times)
Vince
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« on: September 18, 2012, 12:44:56 PM »

I have a couple of questions regarding the vacuum advance and the hose hook-ups for a 1969 Z/28 with the 30-30 cam.  I had the valve lash settings done using JohnZ's guide from this website.  My cars initial and mechanical advance settings have been set to 11 degrees initial and 36 total all in at 2,800 rpm.  My car currently does not have vacuum advance hooked up, and I want to set it up as JohnZ describes in his article using manifold vacuum. 

First question:  When I set my car up like this the idle went up to around 1,400-1,500 rpm.  Is this normal?  Without the vacuum advance it is between 900-1,000.  Is all I have to do is adjust the idle speed to 900-1,000 with the manifold vacuum hook-up?

Second question:  What do I do with the nipple on the AIR diverter valve?  As far as I know my AIR system is operational.  Can I leave the nipple exposed or should I put a plug over it?  Is their something else I need to do regarding the AIR system?

Thank you.
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Stingr69
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« Reply #1 on: September 18, 2012, 02:47:28 PM »

Lower the throttle to get to about 950 RPM. The rise in RPM when you go to manifold vacuum is expected. 

If your diverter valve still holds vacuum, you might keep it functional. I would not. The vacuum tap on the carb should not be allowed to leak. If it is not used for anything else (choke pull off?) you need to cap it.  The nipple on the diverter valve does not need to be pluged.

-Mark.
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JohnZ
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« Reply #2 on: September 19, 2012, 10:14:48 AM »

Second question:  What do I do with the nipple on the AIR diverter valve?  As far as I know my AIR system is operational.  Can I leave the nipple exposed or should I put a plug over it?  Is their something else I need to do regarding the AIR system?

Thank you.

If your A.I.R. system is operational, note that plugging or disconnecting the vacuum signal line to the diverter valve places the valve in the open position by default (the spring on the diaphragm holds the valve open with no vacuum on it), so air from the pump is injected into the exhaust manifolds at all times; this can result in "popping" from the exhaust pipes on over-run. With the vacuum signal line connected and the valve diaphragm functioning normally, the valve closes on over-run (high vacuum signal), and the air from the pump is diverted through the diverter valve muffler instead of into the exhaust manifolds. If the "popping" bothers you, just leave the diverter valve connected so the system operates normally (my pump is "gutted", so that's not an issue on my car).

See page 6T-2, Figure 4 in the '69 Chassis Service Manual to see how the diverter valve works; with vacuum, it's closed and dumps air through the muffler, and without vacuum, it's open and air goes to the exhaust manifolds.
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'69 Z/28
Fathom Green
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Vince
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« Reply #3 on: September 19, 2012, 01:17:19 PM »

Thanks guys for your info.
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