Low vacuum 10 at idle
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if you are on a ported vacuum port no, you are getting vacuum above the throttle plate and wont see any change until you motor is sucking alot of air, manifold vacuum is below the plate and is directly effected by the cylinder vacuum.
heres some articles if my explanation isnt the greatest
Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it
and this one
Vacuum Advance
This article is about the vacuum advance as far as how it is related to the carburetor. There is many good articles about how the vacuum advance works. There is not much information about how the carburetor enters into this picture.
Ported vs Non Ported
The Non Ported vacuum on the carburetor feeds vacuum directly from below the throttle plate, or intake manifold vacuum.For the stock built engine the non ported vacuum advance is the most efficient way to go. However if your engine has all the smog controls that went along with the ported type of carburetor, then you need to stick to that type.
Ported vacuum feeds vacuum from above the throttle plate. This means no vacuum at idle (because the throttle plate is closed). This type of vacuum advance was found with engines that used low initial timing such as 2-4 degrees. This was the 1st attempt at reducing hydrocarbons prior to the catalytic converter.
How to tell if a carburetor is ported.Follow the vacuum passage with compressed air to see where then vacuum originates. If it originates below the throttle plate, then you have a non ported carburetor. If it originates above the throttle plate, then you have a ported carburetor.
In general most ported vacuum advance carburetors will be found on 1970 & later carburetors.
Are you having idle problems?You might have a ported vacuum advance type of carburetor, when you actually should be using a non ported type. This might happen when you install a late model carburetor (ported) on a pre 1970 engine.
Edelbrock with 2 vacuum advance ports.Looking at the front the left vacuum advance is ported. This should be used on any vehicle that has smog control such as a vacuum canister.On more basic engine installs use the right port, which is a non ported source
heres some articles if my explanation isnt the greatest
Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it
and this one
Vacuum Advance
This article is about the vacuum advance as far as how it is related to the carburetor. There is many good articles about how the vacuum advance works. There is not much information about how the carburetor enters into this picture.
Ported vs Non Ported
The Non Ported vacuum on the carburetor feeds vacuum directly from below the throttle plate, or intake manifold vacuum.For the stock built engine the non ported vacuum advance is the most efficient way to go. However if your engine has all the smog controls that went along with the ported type of carburetor, then you need to stick to that type.
Ported vacuum feeds vacuum from above the throttle plate. This means no vacuum at idle (because the throttle plate is closed). This type of vacuum advance was found with engines that used low initial timing such as 2-4 degrees. This was the 1st attempt at reducing hydrocarbons prior to the catalytic converter.
How to tell if a carburetor is ported.Follow the vacuum passage with compressed air to see where then vacuum originates. If it originates below the throttle plate, then you have a non ported carburetor. If it originates above the throttle plate, then you have a ported carburetor.
In general most ported vacuum advance carburetors will be found on 1970 & later carburetors.
Are you having idle problems?You might have a ported vacuum advance type of carburetor, when you actually should be using a non ported type. This might happen when you install a late model carburetor (ported) on a pre 1970 engine.
Edelbrock with 2 vacuum advance ports.Looking at the front the left vacuum advance is ported. This should be used on any vehicle that has smog control such as a vacuum canister.On more basic engine installs use the right port, which is a non ported source
Had a 1976 L-82, 4-sp
Joined: Mar 2011
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From: Some days your the dog and some days your the hydrant.
Royal Canadian Navy
10" of vacuum is way low for a stock L-82. It should be up around 20" on a ported vacuum source as soon as the throttle is cracked open. Connect your vacuum gauge directly to a manifold vacuum source and see what you get.
Had a 1976 L-82, 4-sp
Joined: Mar 2011
Posts: 5,338
Likes: 1,213
From: Some days your the dog and some days your the hydrant.
Royal Canadian Navy
If you don't have a working emissions system, then port vacuum, which is part of the emissions system, would be pointless to use. So, I would use manifold vacuum. 10" at idle is low. Have you checked your ignition timing? It could be retarded too much due to a loose dist'r clamp bolt or worn parts but the HEI is a good dist'r just the same and worth keeping, if that's the problem. Try using manifold vacuum and 14* initial timing and see how it runs.
Last edited by resdoggie; May 14, 2021 at 10:09 AM.
Cruising
Joined: Apr 2021
Posts: 13
Likes: 7
From: North Carolina
Unplug vacuum advance and check/set ignition timing. Low timing can cause low vacuum. The vacuum advance should be plumbed to live manifold vacuum-not ported vacuum. There is the possibility that the balancer has spun and timing can’t be checked/set and is incorrect. Check for stuck PCV valve.
