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I bought a vacuum meter today and watched a couple videos specific to adjusting idle mixture on a quadrajet with the meter. I am a bit perplexed regarding what I thought were manifold vacuum connections. It is my understanding the manifold connections should pull vacuum at idle and ported when throttle is applied. There are two connections at the bottom of the carburator below the gasket on either side of the pcv connection the videos said were manifold connections, but the gauge read zero unless I applied throttle which gave a reading of about 15 inches. I had to connect to the pcv connection which was at 19 inches from my ear adjustment. I was able to get the vacuum to about 21 inches with the gauge. This seems high considering the green zone on the gauge ends at 20. Is there a problem with 21 inches? What about the connections below the gasket being ported?
You could connect it to a port on the intake. 21 is about the most you can get out of a stock engine. I'm not concerned about the amount, I look for a steady needle.
What about the connections below the gasket being ported?
This is one of those instances when identifying the model number of the Q-Jet can be important, in addition sometimes the engine application and year of application. "Rochester" made many models and variations of the Q-Jet, but so did Carter & Holley. Basically, there are three areas of the Q-Jet carburetor: (1) the air horn located on top (2) the main fuel bowl located in the middle and (3) the base plate (what some refer to as the throttle body). NOTE: There are x3 gaskets: one (the top gasket) separates the air horn from the main (body) fuel bowl, moving downwards is the second gasket located between the bottom of the main (body) fuel bowl and the base plate; finally, there is a gasket located at the bottom of the base plate where the carburetor sits on the intake manifold.
In "GENERAL": (1) any port located above the main (body) fuel bowl (ports located within the confines of the air horn) are filtered air sources - no vacuum; (2) ports located within the confines of the main (body) fuel bowl are considered "ported" vacuum sources; and, (3) ports located below the main (body) fuel bowl (location of second gasket) is considered a manifold vacuum source. In nearly every instance, any port located below the main (body) fuel bowl is considered a manifold vacuum source. The only exceptions I'm aware of involve ports located w/in the main(body) fuel bowl in select Rochester Q-Jets. While GENERALLY they are all ported vacuum sources, there are models which have a single manifold vacuum port located within the main (body) fuel bowl containment area located in the front of the Q-Jet. There is a drilled passage from this port directly to manifold vacuum and it was used for thermostatic air cleaner on that pig-nose snout on the air cleaner. This (manifold) vacuum port is for a thermostatic air cleaner.
Therefore, assuming your statement regarding "connections below the gasket being ported" I assume you're referring to the gasket which separates the base plate from the main (body) fuel bowl - those ports are considered manifold vacuum. What we don't know is to which of the various ports you currently have the vacuum hoses connected to. A picture would assist. Note that many owners change the location of vacuum hoses dependent on various carburetors, the application and any modifications. So, identifying where a PCV valve connection is located is not meaningless, but it requires a little more in terms of model number of carburetor, intake manifold, modifications and equipment configured. I didn't mean to muddy the waters, but there are various configurations for Rochester Q-Jets. When using the vacuum gauge, ensure all ports are capped/sealed.
Last edited by Vintage Chief; January 8th, 2022 at 03:43 PM.
In "GENERAL": (1) any port located above the main (body) fuel bowl (ports located within the confines of the air horn) are filtered air sources - no vacuum; (2) ports located within the confines of the main (body) fuel bowl are considered "ported" vacuum sources; and, (3) ports located below the main (body) fuel bowl (location of second gasket) is considered a manifold vacuum source. In nearly every instance, any port located below the main (body) fuel bowl is considered a manifold vacuum source. The only exceptions I'm aware of involve ports located w/in the main(body) fuel bowl in select Rochester Q-Jets. While GENERALLY they are all ported vacuum sources, there are models which have a single manifold vacuum port located within the main (body) fuel bowl containment area located in the front of the Q-Jet. There is a drilled passage from this port directly to manifold vacuum and it was used for thermostatic air cleaner on that pig-nose snout on the air cleaner. This (manifold) vacuum port is for a thermostatic air cleaner.
I can't remember the carburator number, but I looked it up when I got the numbers matching replacement. It's to a 1973 455. It may be a 7043251-I remember it ended in a 1. The ports are located directly above the thick intake manifold gasket and below the gasket to the main body. The PCV port is the larger port between the two that drew no vaccine. They at the bottom of the carburator. They had zero vacuum until I applied throttle.
The ports are located directly above the thick intake manifold gasket and below the gasket to the main body. The PCV port is the larger port between the two that drew no vaccine. They at the bottom of the carburator. They had zero vacuum until I applied throttle.
From what you've described "located directly above the thick intake manifold gasket and below the gasket to the main body" those are manifold vacuum ports. Something is amiss as those ports should supply constant manifold vacuum at idle. (I understand you meant vacuum instead of vaccine).
I'm not spot-on familiar with the 7043251 (which is correct for a 1973 455), but here's a write-up on that carburetor. If you review the pictures in the first post by the OP (the little pictures which you can zoom), it appears the two ports you're referring to are ported vacuum sources - at least the one that is capped on the driver's side is ported, I can't see the one where the automatic transmission modulator valve vacuum source is located. If that is the case, then they should have no vacuum at idle and increase in vacuum as the throttle blades open since they're ported vacuum ports. Maybe I can find another picture of a 7043251.
From what you've described "located directly above the thick intake manifold gasket and below the gasket to the main body" those are manifold vacuum ports. Something is amiss as those ports should supply constant manifold vacuum at idle. (I understand you meant vacuum instead of vaccine).
unfortunately that is a very frequently used word in autocorrect. Yeah I couldn't explain it or how it would be possible. Both ports are plugged so hopefully it's not an issue. The car seems to run fine.
unfortunately that is a very frequently used word in autocorrect. Yeah I couldn't explain it or how it would be possible. Both ports are plugged so hopefully it's not an issue. The car seems to run fine.
LOL No worries. I can't find a front looking profile picture of both of those ports, but the one is surely ported, most likely they're both ported - not sure. Since they're plugged and non-functioning there's no worries. Based upon your vacuum readings they're both ported vacuum sources and the videos you reviewed were incorrect - they're not manifold vacuum they're ported vacuum.
EDIT: I'd suggest you take one more final look at the port locations though. If they're ported, as illustrated by your vacuum gauge readings, note the location of the ports. The one port (on the right) in the link I provided to you is located ABOVE the base plate (it is capped in the picture). That is a ported vacuum as you can see it's located above the base plate (e.g. above the throttle blades) and will only provide vacuum when the throttle blades begin to open. Cheers!
Last edited by Vintage Chief; January 8th, 2022 at 05:56 PM.
Finally, here's a good description of the ports on your 7043251 (455), which is same as 7043250 (350) - the jetting/needles are different is all. Note that Port #3 is manifold vacuum (as it resides below the throttle blades).
Finally, here's a good description of the ports on your 7043251 (455), which is same as 7043250 (350) - the jetting/needles are different is all. Note that Port #3 is manifold vacuum (as it resides below the throttle blades).
well something isn't right... That looks exactly like my carburator, and on mine, ports 1 and 3 are ported. At first I thought the gauge was not working until I gave it some gas and they pulled 15 inches.
I found another 7043250 (350) same as your 7043251 (455) with a better view. As best I can tell, I have them labeled correctly unless someone states otherwise.
The best I can muster up is someone elected to use the hole identified with question marks as a ported vacuum source? I have no idea if that can be done or was done but it's about all I have at this point.
The best I can muster up is someone elected to use the hole identified with question marks as a ported vacuum source? I have no idea if that can be done or was done but it's about all I have at this point.
that's alright. I wish one of them was a manifold vacuum as the picture above indicates. I just read an interesting post on another site from a GM designer who worked on vacuum advance. He said vacuum advance prior to emissions was hooked up to manifold vacuum to advance timing to keep the engine cool under the lean condition at idle speed. The move to ported vacuum for the advance was done as a bandaid fix for emissions to help burn off exhaust gas and was not good for engine temperature at idle. He was pretty adamant that advance should be connected to manifold vacuum as originally intended. Maybe I can tee off the transmission line.
that's alright. I wish one of them was a manifold vacuum as the picture above indicates. I just read an interesting post on another site from a GM designer who worked on vacuum advance. He said vacuum advance prior to emissions was hooked up to manifold vacuum to advance timing to keep the engine cool under the lean condition at idle speed. The move to ported vacuum for the advance was done as a bandaid fix for emissions to help burn off exhaust gas and was not good for engine temperature at idle. He was pretty adamant that advance should be connected to manifold vacuum as originally intended. Maybe I can tee off the transmission line.
We all learn a slurry of the "Oh ****" I didn't know that from time-to-time. I can't say I'm an expert on carburetors. If I was holding the carburetor in my hand and I saw (then validated by blowing air through) the channel to that particular port I could determine ported or manifold. If you can apply a vacuum gauge to that one specific port and validate no vacuum at idle and increased vacuum by opening the throttle blades, I would surmise it was a ported vacuum source (but WTH do I know - I didn't engineer it).
I was able to get the vacuum to about 21 inches with the gauge. This seems high considering the green zone on the gauge ends at 20. Is there a problem with 21 inches?
No, there is no problem with that. Higher vacuum is a good thing. I was able to tune my 1970 Supreme 350-4bbl with a 204* cam to 21” back in the 80s.
So why not connect your vacuum gauge to one of the intake manifold ports so there’s no ambiguity about it being a full manifold source?
I’ve been looking all over for a PICTURE (not a diagram) of the intake manifold on my 350 with a Rochester quad. Thanks to the reply to Andy’s posting, I found exactly what I was looking for. This site has been an invaluable resource for me. I don’t even have to ask a question... The answer is always on the site somewhere. Thanks Classic Oldsmobile.
January 8th, 2022, 02:20 PM
