General Category > Quadrajet Carb Talk and Tips
What are these holes for?
Cadman-iac:
Yesterday I was able to work on my carburetor, I raised the float level to 9/32" from 13/32", which did improve the AFR over most of the range, but it still has a lean spot just off-idle.
The idle mixture screws will change the AFR from extremely lean to extremely rich, and once the engine speed is high enough to pull in the main circuit it runs more on the rich side, it's that transition from idle circuit to main that seems to be the problem.
novadude:
Just curious... what size fuel inlet (needle seat)? The Doug Roe book explains that float height and inlet size work together to set the actual fuel height in the bowl. The lean spot just off idle... is it still a steady state lean spot, or just transition? If it is transition, maybe try re-seating your accelerator pump check ball? Look to see if you get a nice pump shot the instant the throttle moves.
novadude:
I finally read through all the AI stuff on bleeds... I think AI is confusing quadrajet with an air-bled nozzle style carb like a Holley. A lot of what is said there doesn't seem to make sense. All of the Q-jet bleeds are not bleeding air into the main well below fuel level like a Holley. I am not super clear on how the relationship between the three main air bleeds works. Seems to me like you have the upper bleed which is at air horn pressure (atmospheric), the lower main air bleed in the body which would probably be at a lower pressure as air flow increases through the carb.
Then you have the nozzle bleed which is at venturi pressure. Since this would be at a lower pressure than the upper main air bleed, it would seem that air would actually flow from the main air bleeds into this bleed and blow into the venturi beside the fuel entering the venturi through the brass nozzle. Look close... your 0.055 bleed does not actually extend into the brass fuel nozzle, so I'd be very careful trying to reduce this with setscrews. Setscrew would need to be a perfect length so as not to seat on the brass nozzle so that air would still pass through. I wouldn't attempt it.
Seems like maybe more transfer slot fuel might help... how about a larger IDCR? I'm trying to recall... did you ever go bigger that 0.046 with your current idle air bleed configuration? Seems to me that your stock-ish engine shouldn't need it, but that is probably what I'd try next.
Cadman-iac:
--- Quote from: novadude on November 03, 2025, 11:39:24 AM ---Just curious... what size fuel inlet (needle seat)? The Doug Roe book explains that float height and inlet size work together to set the actual fuel height in the bowl. The lean spot just off idle... is it still a steady state lean spot, or just transition? If it is transition, maybe try re-seating your accelerator pump check ball? Look to see if you get a nice pump shot the instant the throttle moves.
--- End quote ---
The inlet size is .130" I think, or .135", I know it's the biggest one, I got it from Cliff with the kit, so it's getting good fuel flow. The regulator is set at 5.5lbs, and it's rock steady, so there's plenty of fuel available as well.
By "steady state" if you mean it doesn't suddenly appear and just as quickly disappear but it holds steady at a given rpm range, yes, it's a steady state problem. I can give it throttle and it'll go away when the rpm goes up.
The accelerator pump definitely gives a good squirt, you can see the AFR go rich immediately, as low as 10.0 before it comes back up to a more normal range, normal being around 13.5-14.5.
The lean condition is fleeting if you're getting on it, it passes through the range quickly, but if you're steady on the throttle it'll stay in that lean spot until you accelerate past it.
Cadman-iac:
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.
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