Vacuum Advance Problems
Thread Starter
5th Gear
Joined: Nov 2005
Posts: 5
Likes: 0
From: Ohio "Never Give Up"
Vacuum Advance Problems
I have a 78 silver anniv. bone stcok, with a Q-jet carb 17059222.
After setting the initial advance, I went to plug the vacuum advance back in and it advanced the engine immediately, the full 20deg. I checked all avail ported vacuum ports on the carb, each one is pulling around 12 lbs. It goes to full vac advance at idle knowmatter which port its plugged into.
Here is a picture of the carb setup. The Vacuum Advance hose is noted at both ends with the letter 'A'.
Can anyone help me figure out why it goes to full vacuum advance at idle??
After setting the initial advance, I went to plug the vacuum advance back in and it advanced the engine immediately, the full 20deg. I checked all avail ported vacuum ports on the carb, each one is pulling around 12 lbs. It goes to full vac advance at idle knowmatter which port its plugged into.
Here is a picture of the carb setup. The Vacuum Advance hose is noted at both ends with the letter 'A'.
Can anyone help me figure out why it goes to full vacuum advance at idle??
Drifting
Joined: Jun 2005
Posts: 1,950
Likes: 498
From: Chino Hills CA
Did you set your timing with or without the vacuum advance hooked up, and did you set it at full advance to about 35 degrees BTDC at 2800-3000 rpm? Obviously if you're pulling vacuum at idle, it's not a ported vacuum source. It's hard to see on your picture where the line is going in your carb. Ported vacuum should be just above the throttle plate, zero vacuum at idle. Manifold vacuum is below the throttle plate.
Le Mans Master
Joined: Sep 2001
Posts: 7,739
Likes: 630
From: Unreconstructed, South Carolina
replace vac can
Inside the vac can is a rubber diaphragm and a calibrated coil spring; neither are visible or servicable. The spring provides the majority of opposing force the vacuum signal pulls against. Suggest spring has probably broken. If so, replace the vac can ... easy & cheap $ fix.
Thread Starter
5th Gear
Joined: Nov 2005
Posts: 5
Likes: 0
From: Ohio "Never Give Up"
Originally Posted by lowbuck72
Ported vacuum should be just above the throttle plate, zero vacuum at idle. Manifold vacuum is below the throttle plate.
That's the thing...all ports above the throttle plate are pulling about 12lbs of vacuum..mabye the problem lies within the carb.
Tech Contributor
Joined: Aug 1999
Posts: 14,370
Likes: 6,364
From: At my Bar drinking and wrenching in Lafayette Colorado
Originally Posted by chianesi
Can anyone help me figure out why it goes to full vacuum advance at idle??
Your car uses full manifold vacuum to the distributor through a thermal switch. The car will run best in this configuration. Ported vacuum on your car is used only for the EGR. The ported vacuum nipple on that carb is located in the lower throttle plate on the forward passenger side of the carb, sticking out at a 45-degree angle. That ported source should be connected to the EGR and not the distributor. All other ports on your carb are manifold vacuum - including the one to your distributor.
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