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Quadrajet Carb Talk and Tips / Re: What are these holes for?
« Last post by Cadman-iac on November 03, 2025, 06:51:25 PM » About the AI information on the Q-Jet, I had to carefully word my questions when I was looking for information, because it definitely refers to the Holley if you don't specifically ask about the Rochester QuadraJet. I had to ask several different ways to get what I did, and I noticed that even if you ask the same question again you get a different answer in that the information is the same but it's formatted differently. Kinda like a kid doing a book report and they change the wording to avoid any appearance of plagiarism, at least that's how I see it anyway.
As far as if it's correct, I'm not sure. It makes sense, other than the air going through the nozzle air bleed, as I see it, must come through the upper primary air bleeds. The nozzle bleed is buried in the main metering well with the only access being the upper bleed and the fuel well itself, unless I'm missing something somewhere. I would agree with you on this.
On where the nozzle air bleed exits the main fuel wells, it comes out above the casting that the nozzles are pressed into, just slightly. There is also a very small gap all the around the top half of the nozzle that allows air to flow around/past them.
I took an old core and practiced drilling and tapping the bleed holes, and yes, I was careful to not go too far, but to resize the hole you do have to go all the way through the casting, but not the actual brass nozzle piece, otherwise you still have a small section with the original size in it yet.
I have some set screws coming tomorrow as well as a couple of taps. I'm not going to try it on the carb I've got on the truck, not until I'm positive that there's no other way to correct this issue.
I have been thinking about making the bypass air resizable with drilled set screws, (the base plate has the bypass air going into the short channels on the sides of the primary bores instead of coming out right into the bores so it'll be easier to drill), and maybe making the LIAB holes bigger for the bigger set screws, but I'll have to try it out on a junk core first to be sure it'll work.
I did run a bigger DCR of .057" and even a .061, but the .061 didn't change anything.
Now that I can change things without damaging the carb, I'm going to try different sizes for the UIAB'S, the DCR'S, and see how it responds. If it doesn't work I can go back and try something else.
I will figure this out, I may be bald before I do, but I'm not going to quit.
Not sure how this plays into the equation, but I did remove the POE tubes from the air horn in my effort to make this carburetor match the specs of a Chevrolet carb, with the exception of the nozzle air bleeds.
I was thinking about it, before I raised the float level, when I ran it without the POE tubes it was running lean throughout the operating range, and when I went up one jet size it improved throughout the range.
I think they were using the POE system to augment the main system.
As far as if it's correct, I'm not sure. It makes sense, other than the air going through the nozzle air bleed, as I see it, must come through the upper primary air bleeds. The nozzle bleed is buried in the main metering well with the only access being the upper bleed and the fuel well itself, unless I'm missing something somewhere. I would agree with you on this.
On where the nozzle air bleed exits the main fuel wells, it comes out above the casting that the nozzles are pressed into, just slightly. There is also a very small gap all the around the top half of the nozzle that allows air to flow around/past them.
I took an old core and practiced drilling and tapping the bleed holes, and yes, I was careful to not go too far, but to resize the hole you do have to go all the way through the casting, but not the actual brass nozzle piece, otherwise you still have a small section with the original size in it yet.
I have some set screws coming tomorrow as well as a couple of taps. I'm not going to try it on the carb I've got on the truck, not until I'm positive that there's no other way to correct this issue.
I have been thinking about making the bypass air resizable with drilled set screws, (the base plate has the bypass air going into the short channels on the sides of the primary bores instead of coming out right into the bores so it'll be easier to drill), and maybe making the LIAB holes bigger for the bigger set screws, but I'll have to try it out on a junk core first to be sure it'll work.
I did run a bigger DCR of .057" and even a .061, but the .061 didn't change anything.
Now that I can change things without damaging the carb, I'm going to try different sizes for the UIAB'S, the DCR'S, and see how it responds. If it doesn't work I can go back and try something else.
I will figure this out, I may be bald before I do, but I'm not going to quit.
Not sure how this plays into the equation, but I did remove the POE tubes from the air horn in my effort to make this carburetor match the specs of a Chevrolet carb, with the exception of the nozzle air bleeds.
I was thinking about it, before I raised the float level, when I ran it without the POE tubes it was running lean throughout the operating range, and when I went up one jet size it improved throughout the range.
I think they were using the POE system to augment the main system.

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