How much vacuum at idle?
Thread Starter
Pro
Joined: Aug 2002
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From: Southeast WI
How much vacuum at idle?
Ok, I tried to do a basic tune-up this past weekend. Checked compression in each cylinder, dwell, timing, idle mixture and speed. The car runs better now, so I know I did something right, but I also noticed something odd.
Is there supposed to be vacuum pulling on the vacuum advance by the distributor at engine idle? I measured ~5" Hg at idle off of this port on the carb. It has to be the right port because the two other ports on the carb, which are capped, are pulling much harder.
By the way, this is on a standard Rochester Q-Jet 4MV.
Your advice would be appreciated.
Thanks!
Is there supposed to be vacuum pulling on the vacuum advance by the distributor at engine idle? I measured ~5" Hg at idle off of this port on the carb. It has to be the right port because the two other ports on the carb, which are capped, are pulling much harder.
By the way, this is on a standard Rochester Q-Jet 4MV.
Your advice would be appreciated.
Thanks!
Thread Starter
Pro
Joined: Aug 2002
Posts: 581
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From: Southeast WI
Forgive my ignorance, but that doesn't seem right to me. At idle there is maximum vacuum in the intake manifold, right? I don't want that much vacuum going to the vacuum advance cannister at idle, do I?
Race Director
Joined: Dec 2004
Posts: 15,030
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Yes, you do.
At idle, the vacuum pulling on the vacuum advance keeps the timing at whatever you have it set at. When you step on the throttle, there is a momentary loss of vacuum which, releasing the pull on the vacuum advance, retarding the timing. Once the vacuum catches up, full timing is restored.
At idle, the vacuum pulling on the vacuum advance keeps the timing at whatever you have it set at. When you step on the throttle, there is a momentary loss of vacuum which, releasing the pull on the vacuum advance, retarding the timing. Once the vacuum catches up, full timing is restored.
Race Director
Joined: Dec 2004
Posts: 15,030
Likes: 2
A healthy, stock engine will pull anywhere from 16 to 21 in. Of course, mods such as cam, etc are going to affect how much vacuum you are pulling.
I just bought an '80 that has had engine mods, and I'm pulling 10
Naturally the headlights won't open! Gonna have to fix that...
I just bought an '80 that has had engine mods, and I'm pulling 10
Naturally the headlights won't open! Gonna have to fix that...
Race Director
Joined: Sep 1999
Posts: 11,361
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From: Plano TX
Nitronick: Just as a side note; I measured how much vacuum is needed to open the headlight by using a vacuum pump. It took no more than 2" so there is something wrong with your vacuum system. Now the brakes are going to be pretty hard at only 10"...
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Thread Starter
Pro
Joined: Aug 2002
Posts: 581
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From: Southeast WI
Guys,
I'm at home now and I've consulted the '69 Service Manual. In the back section with all the miscellaneous technical specs, it lists a couple of specs for Vacuum Advance (In Crank Degrees).
0 Degrees @ 8" Hg
15 Degrees @ 15.5" Hg
Can someone please explain what these specs mean?
I'm at home now and I've consulted the '69 Service Manual. In the back section with all the miscellaneous technical specs, it lists a couple of specs for Vacuum Advance (In Crank Degrees).
0 Degrees @ 8" Hg
15 Degrees @ 15.5" Hg
Can someone please explain what these specs mean?
Race Director
Joined: Sep 2004
Posts: 11,874
Likes: 263
From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
Originally Posted by Tele_Man
Guys,
I'm at home now and I've consulted the '69 Service Manual. In the back section with all the miscellaneous technical specs, it lists a couple of specs for Vacuum Advance (In Crank Degrees).
0 Degrees @ 8" Hg
15 Degrees @ 15.5" Hg
Can someone please explain what these specs mean?
I'm at home now and I've consulted the '69 Service Manual. In the back section with all the miscellaneous technical specs, it lists a couple of specs for Vacuum Advance (In Crank Degrees).
0 Degrees @ 8" Hg
15 Degrees @ 15.5" Hg
Can someone please explain what these specs mean?
Thread Starter
Pro
Joined: Aug 2002
Posts: 581
Likes: 0
From: Southeast WI
Thanks for explaining those specs.
I ran my car tonight and got the engine to operating temperature, then I came back home, pulled it into the stable and kept the engine idling. I found three vacuum ports off the Q-Jet and measured the vacuum from each of them.
Port 1
Located on the front of the carb on the throttle side toward the bottom. This is the one currently connected to the vacuum advance cannister.
Reading: 10" Hg
Port 2
Located just below the choke off the right side of the carb. This one is capped; not currently connected to anything.
Reading: ~15-16" Hg
Port 3
Located on the rear of the carb toward the top. This one is also capped; not currently connected to anything.
Reading: 0" Hg
Does this seem ok to you guys? Is there something wrong with Port 1? Is it the correct vacuum to pull at warm idle? And should this be the port which is connected to the vacuum cannister?
Thanks!
I ran my car tonight and got the engine to operating temperature, then I came back home, pulled it into the stable and kept the engine idling. I found three vacuum ports off the Q-Jet and measured the vacuum from each of them.
Port 1
Located on the front of the carb on the throttle side toward the bottom. This is the one currently connected to the vacuum advance cannister.
Reading: 10" Hg
Port 2
Located just below the choke off the right side of the carb. This one is capped; not currently connected to anything.
Reading: ~15-16" Hg
Port 3
Located on the rear of the carb toward the top. This one is also capped; not currently connected to anything.
Reading: 0" Hg
Does this seem ok to you guys? Is there something wrong with Port 1? Is it the correct vacuum to pull at warm idle? And should this be the port which is connected to the vacuum cannister?
Thanks!
Race Director
Joined: Sep 2004
Posts: 11,874
Likes: 263
From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
well not to confuse things but see how it rides before you make it permanent and adjust anything...some times it runs well with ported and sometimes it does better with direct....i switched from running ported and went to manifold.
Burning Brakes
Joined: Aug 2002
Posts: 1,146
Likes: 10
From: Scottsboro Alabama
You should ask the timing, carburetor guru's over in C3VR forum. Some of them never heard of using manifold vacuum! Every thing should be hooked up to ported! How to set timing is even crazier! 
