Vacuum Advance and a Rochester Quadrajet
March 23rd, 2011, 01:20 PM
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Location: Tulsa, Ok
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Vacuum Advance and a Rochester Quadrajet
I have some questions in regards to vacuum advance and manifold vacuum.
On my 69 442, my Rochester Quadrajet (29251RJ 0989) has a port for the vacuum advance to attach to.
Should I attach my vacuum advance to this port on the carburetor or directly to the manifold for vacuum? Is the vacuum port on the carburetor considered a ported vacuum. I have very little vacuum on the carburetor port, and when I attach the advance directly to the manifold my idle rises 250 300 rpm. Isnt it better to have more vacuum available to the vacuum advance to allow it to compensate for the lean mixture at idle?
Also, in reading some of the articles on this forum about initial timing, mechanical timing, and vacuum advance, I seem to have gotten lost on how to determine the actual vacuum advance timing. If I understand this correctly, to determine initial timing disconnect the vacuum advance. To set mechanical advance adjust the springs and weights inside the distributor to have full advance to desired setting by roughly 3,000 rpm. How do you determine the vacuum advance timing?
What are some good numbers to shoot for on the advance, mechanical and initial timing?
On my 69 442, my Rochester Quadrajet (29251RJ 0989) has a port for the vacuum advance to attach to.
Should I attach my vacuum advance to this port on the carburetor or directly to the manifold for vacuum? Is the vacuum port on the carburetor considered a ported vacuum. I have very little vacuum on the carburetor port, and when I attach the advance directly to the manifold my idle rises 250 300 rpm. Isnt it better to have more vacuum available to the vacuum advance to allow it to compensate for the lean mixture at idle?
Also, in reading some of the articles on this forum about initial timing, mechanical timing, and vacuum advance, I seem to have gotten lost on how to determine the actual vacuum advance timing. If I understand this correctly, to determine initial timing disconnect the vacuum advance. To set mechanical advance adjust the springs and weights inside the distributor to have full advance to desired setting by roughly 3,000 rpm. How do you determine the vacuum advance timing?
What are some good numbers to shoot for on the advance, mechanical and initial timing?
March 23rd, 2011, 04:13 PM
#2
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Join Date: Oct 2009
Location: Poteau, Ok
Posts: 40,771
I you have points make sure your dweel is at 30 deg before setting timing. When setting your timing its with the vacuum adv disconnected.
The vacuum adv can be connected to manifold vacuum and as you noticed it will bring the idle up. All you have to do is adj your curb idle screw down to the desired rpm with the adv connected.
The vacuum adv will add approx 12-14 deg btdc. You can measure this by checking the timing with the line connected vs with the line disconnected at idle and subtract the value of the non connected reading from the connected.
Your mechanical timing @ 3000-3500 rpm with vacuum disconnected is your total mechanical adv, it should be somwhere around 32 - 36 deg btdc. and with your vacuum adv connected should be around 50-55 deg btdc @ 4000 rpm.
I hope this helps.
The vacuum adv can be connected to manifold vacuum and as you noticed it will bring the idle up. All you have to do is adj your curb idle screw down to the desired rpm with the adv connected.
The vacuum adv will add approx 12-14 deg btdc. You can measure this by checking the timing with the line connected vs with the line disconnected at idle and subtract the value of the non connected reading from the connected.
Your mechanical timing @ 3000-3500 rpm with vacuum disconnected is your total mechanical adv, it should be somwhere around 32 - 36 deg btdc. and with your vacuum adv connected should be around 50-55 deg btdc @ 4000 rpm.
I hope this helps.
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