Carburetor (timed vacuum vs. full vacuum advance?)
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Instructor
Joined: May 2004
Posts: 167
Likes: 4
From: Glenmont NY
Carburetor (timed vacuum vs. full vacuum advance?)
Do Corvettes have a standard set-up when it comes to timed vacuum or full vacuum advance? What does the vacuum advance do for the car in the long run?
The instructions for my carb install tell me to determine between the two, but since the car doesn't run with the current carb, I can't start it and "feel" for vacuum, I don't know which vacuum port on my Edelbrock 1406 to connect it to.
The instructions for my carb install tell me to determine between the two, but since the car doesn't run with the current carb, I can't start it and "feel" for vacuum, I don't know which vacuum port on my Edelbrock 1406 to connect it to.
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Joined: May 2003
Posts: 72
Likes: 5
From: Wilmington DE
Which Vacuum to Use
What you refer to as "timed" vacuum I think is more commonly called "ported". Ported vacuum is zero at idle, but comes in when you open the throttle. To see the effect, get an inexpensive vacuum test gauge and run a hose from the carburetor to the passenger compartment. Drive around and watch how the different ports affect the gauge.
My car tends to surge a bit more at very low engine speeds with the throttle closed and with the distributor connected to full vacuum, say turning into a driveway, so I am using ported vacuum. Try it both ways and see which you like better. You will have to adjust the idle stop when switching ports.
The purpose of vacuum advance is for gas mileage. A car gets better mileage with lots of advance, but too much advance causes preignition at large throttle openings, so the vacuum advance mechanism gives more advance at small throttle opening (high vacuum), but shuts off and leaves advance control to the mechanical advance at larger throttle opening. On my big block '71, this is worth about 1 mpg. Doesn't sound like much, but it is a 10% increase over no vacuum advance.
Since your car is not running yet, hook the vacuum advance to either port or leave it disconnected. It doesn't matter that much. Play around with it after you get the car running right.
My car tends to surge a bit more at very low engine speeds with the throttle closed and with the distributor connected to full vacuum, say turning into a driveway, so I am using ported vacuum. Try it both ways and see which you like better. You will have to adjust the idle stop when switching ports.
The purpose of vacuum advance is for gas mileage. A car gets better mileage with lots of advance, but too much advance causes preignition at large throttle openings, so the vacuum advance mechanism gives more advance at small throttle opening (high vacuum), but shuts off and leaves advance control to the mechanical advance at larger throttle opening. On my big block '71, this is worth about 1 mpg. Doesn't sound like much, but it is a 10% increase over no vacuum advance.
Since your car is not running yet, hook the vacuum advance to either port or leave it disconnected. It doesn't matter that much. Play around with it after you get the car running right.
Intermediate
Joined: Jan 2005
Posts: 32
Likes: 3
From: Rock Spring GA
When looking at the front of the carb, the ported vac is to the left, the pcv in the center and manifold vac to the right. After tuning the car per Lars, my 76 runs/performs better with the manifold vac connected and the ported blocked.
