Vacuum advance question
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Freshman User
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Vacuum advance question
Hi all. I have a quick vacuum advance question. I have a 79 f250 with a 351M with an edelbrock carb. The vacuum hose from the vacuum advance is gone. When I replace it, where do I run it to? If your looking at the front of the carb, I have a capped vacuum port on the left side. Thanks in advance.
Elder User
Joined: Sep 2008
Posts: 563
Likes: 0
From: DeKalb, Ill.
That carb port should work.
While it's running, pull that cap and check it for vacuum.
A gauge is always best, then you'll now what kind of numbers you have.
Also, when you hook up the hose, watch the advance mechanism.
If it's functioning, you should see it move the instant you hook the hose to it.
Murph.
While it's running, pull that cap and check it for vacuum.
A gauge is always best, then you'll now what kind of numbers you have.
Also, when you hook up the hose, watch the advance mechanism.
If it's functioning, you should see it move the instant you hook the hose to it.
Murph.
Junior User
Joined: May 2009
Posts: 77
Likes: 1
Typically, the left side vacuum port is just a little HIGHER than the right side (if you look at the front of the carb dead on, you can see what i mean). The one on the left side is vacuum above the throttle plates and is used for vacuum advance specifically.
the left port will have very little vacuum at idle, due to the fact that the throttle plates are closed. When the throttle plates open up, then you should feel some vacuum on the left port.
I hope i've got it right here, this is how i understand how it works.
JT
the left port will have very little vacuum at idle, due to the fact that the throttle plates are closed. When the throttle plates open up, then you should feel some vacuum on the left port.
I hope i've got it right here, this is how i understand how it works.
JT
Elder User
Joined: Sep 2008
Posts: 563
Likes: 0
From: DeKalb, Ill.
Typically, the left side vacuum port is just a little HIGHER than the right side (if you look at the front of the carb dead on, you can see what i mean). The one on the left side is vacuum above the throttle plates and is used for vacuum advance specifically.
the left port will have very little vacuum at idle, due to the fact that the throttle plates are closed. When the throttle plates open up, then you should feel some vacuum on the left port.
I hope i've got it right here, this is how i understand how it works.
JT
the left port will have very little vacuum at idle, due to the fact that the throttle plates are closed. When the throttle plates open up, then you should feel some vacuum on the left port.
I hope i've got it right here, this is how i understand how it works.
JT
If this is true, that there isn't much vacuum at this port with the throttle closed,
that might not be the best place for the vac. advance.
The advance should have a more direct source.
At idle, or cruise when you have strong vacuum pressure, the advance will be "all in".
When the throttle is then opened up, vacuum pressure plummets and the
vacuum advance then retards the timing counteracting the mechanical advance.
Thus, avoiding pre-detonation (the evil "ping").
Even more now, a gauge would be GREAT to have.
Your advance should be hooked up to a port that has very strong vacuum at idle.
If you find a port that reads around 20" of pressure, snap the throttle all the way once.
If that same port drops to near nothing, then returns to 20", that's a good port.
Hope all of this is helping,
Murph.
Junior User
Joined: May 2009
Posts: 77
Likes: 1
OK, I wasn't sure about the position of that carb port.
If this is true, that there isn't much vacuum at this port with the throttle closed,
that might not be the best place for the vac. advance.
The advance should have a more direct source.
At idle, or cruise when you have strong vacuum pressure, the advance will be "all in".
When the throttle is then opened up, vacuum pressure plummets and the
vacuum advance then retards the timing counteracting the mechanical advance.
Thus, avoiding pre-detonation (the evil "ping").
Even more now, a gauge would be GREAT to have.
Your advance should be hooked up to a port that has very strong vacuum at idle.
If you find a port that reads around 20" of pressure, snap the throttle all the way once.
If that same port drops to near nothing, then returns to 20", that's a good port.
Hope all of this is helping,
Murph.
If this is true, that there isn't much vacuum at this port with the throttle closed,
that might not be the best place for the vac. advance.
The advance should have a more direct source.
At idle, or cruise when you have strong vacuum pressure, the advance will be "all in".
When the throttle is then opened up, vacuum pressure plummets and the
vacuum advance then retards the timing counteracting the mechanical advance.
Thus, avoiding pre-detonation (the evil "ping").
Even more now, a gauge would be GREAT to have.
Your advance should be hooked up to a port that has very strong vacuum at idle.
If you find a port that reads around 20" of pressure, snap the throttle all the way once.
If that same port drops to near nothing, then returns to 20", that's a good port.
Hope all of this is helping,
Murph.
If thats the case, the port to the right (the one that is lower) is the vacuum source needed b/c it is more of a manifold vacuum below the throttle plates (vac is highest at idle). I am unfamiliar with any of the vacuum advance setups on these trucks, nor am i familiar with the motorcraft carbs that came on them. I run a total MSD setup (billet dizzy, 6AL Ignition box, Blaster 2 coil) so i am inexperienced when it comes to vacuum advance.
On edelbrock carbs, the left (higher) port is known as a "Ported Vacuum Port", where as the right port (lower) is known as a "Full-time Vacuum Port."
Hopefully someone who knows quite a bit more about vacuum advance can step in here, I don't want to misinform anyone with my inexperience.
JT
*Edit* - some searching on edelbrocks site:
Read 3rd question From bottom on this link
for this video watch "Before you Start Part II" @ just after 1min 45sec he explains the ports
Video
This is my source for my above statements
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
Typically the vacuum advance runs to ported vacuum, which on an Edelbrock is the higher of the two 3/16" vacuum ports on the front of the carb as was mentioned. There are some unique setups that use manifold vacuum; typically these engines specify a much lower base timing number since the advance holds the timing at idle. But in most cases the advance sources from ported vacuum, of which there is none at idle.
Elder User
Joined: Sep 2008
Posts: 563
Likes: 0
From: DeKalb, Ill.
Typically the vacuum advance runs to ported vacuum, which on an Edelbrock is the higher of the two 3/16" vacuum ports on the front of the carb as was mentioned. There are some unique setups that use manifold vacuum; typically these engines specify a much lower base timing number since the advance holds the timing at idle. But in most cases the advance sources from ported vacuum, of which there is none at idle.
Turns out, I only have a good handle on mine. Now I'm curious.
Mine stays advanced until pressure drops below 7".
Then, under a heavy load, I only draw 5" of pressure so my advance
lets loose and retards the timing.
Apparently, the ported system works the other way?
Vacuum pressure is not sent to the advance until the throttle
is opened, under a load. ...right? Then timing is advanced under a load?
What are the applications for these different setups?
Is the latter used for a dual diaphragm setup?
I catch myself in a bubble every now and again, since I learn by working on my truck.
It seems I've done it again.
Murph.
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Post Fiend
Joined: Jul 2004
Posts: 8,786
Likes: 28
From: Northern California
There are Four types of Vacuum advance.
Ported - The Most Comon. No Vacuum advance at idle.
Manifold - Like on Murphs.
A Combination of both- Manifold vacuum at Idle, Ported Vacuum at load.
And after 1980, etc... Computer Controlled - Same as combination, except an additional EGR solenoid bleeds off vacuum if pinging is detected or at a pre set RPM.
Each have a different Base timing proceedure and value, and some are California specific too.
Why matching carbs and distributors together for proper calibration, can be a mess sometimes.
Ported - The Most Comon. No Vacuum advance at idle.
Manifold - Like on Murphs.
A Combination of both- Manifold vacuum at Idle, Ported Vacuum at load.
And after 1980, etc... Computer Controlled - Same as combination, except an additional EGR solenoid bleeds off vacuum if pinging is detected or at a pre set RPM.
Each have a different Base timing proceedure and value, and some are California specific too.
Why matching carbs and distributors together for proper calibration, can be a mess sometimes.
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