Cliff's Quadrajet Parts and Rebuild Kits
General Category => Quadrajet Carb Talk and Tips => Topic started by: Cadman-iac on October 08, 2025, 09:28:27 PM
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While looking at my carburetor I noticed that these holes are not always the same size.
On a Chevrolet, they seem to be .029" in diameter, but on a Cadillac or Oldsmobile they are as big as .055".
What does this do, and how does it effect the AFR ?
Are these an air bleed for the main wells, and why would there be different sizes?
How do they affect the AFR?
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Cliff,
How much does the size of this hole affect your AFR? I don't know what the range is for these, I've seen them from .029" to .055". The .029 being on a Chevrolet carb, the .055 on a Cadillac and an Oldsmobile, I don't have any Pontiac carbs to look at.
I ask because I'm running a Cadillac carb from a 425, #17057230, on a 350 Chevrolet, and I set it up with the recommendations you gave me for a 350 using this carb.
However I keep having a lean condition at certain RPM's and engine loads, and I'm wondering if this hole is a factor.
How do you compensate for the larger hole? This is the main well air bleed, correct?
Rick
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Bumping this to the top, as I am also curious about how these bleeds affect fuel curve.
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Nothing?
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Still nothing?
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Where's Tuner?
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Still curious about how the size of the main nozzle air bleeds affect the AFR, and the relationship between these and the main jet size.
Cliff?
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I couldn't sleep tonight thinking about this carburetor, so I was posing some questions online for AI to answer and here's what I've learned.
In a Rochester Quadrajet carburetor,
the main nozzle air bleed is effectively the final stage of air bleeding, where the air/fuel emulsion is discharged into the venturi, while the upper and lower main circuit air bleeds (also known as emulsion bleeds) are earlier stages that control the initial mixing of air and fuel within the main well.
Difference in Function
Upper and Lower Main Circuit Air Bleeds (Emulsion Bleeds): These bleeds are located within the main well of the carburetor body and air horn. Their primary role is to introduce a calibrated amount of air into the fuel column as it rises from the main jet, creating an emulsion (a mix of air and fuel). This emulsification helps to decrease the fuel's density, promotes better vaporization, and helps "lift" the fuel into the airstream. The size and number of these bleeds shape the fuel curve in the mid-to-high RPM range by varying the air mixing as the fuel level in the well changes with engine speed. The upper bleeds have the most impact, while the lower ones have less effect.Main Nozzle Air Bleed (Discharge Nozzle): The final discharge nozzle (sometimes referred to as the "high-speed bleed" in a general sense, though it's the exit point) is where the air/fuel mixture exits into the boost venturi. While the entire passage acts as an air-bleed type system, the primary function at this point is to discharge the already-emulsified mixture effectively into the high-velocity air stream in the venturi. The final bleed size here fine- tunes the very top-end, full-throttle air/fuel ratio.
Effect of a Larger Main Nozzle Air Bleed
A larger main nozzle air bleed (or increasing the size of the final high-speed bleed restriction if applicable) introduces more air into the system or modifies the discharge pressure characteristics. The general effect is a leaner air/fuel mixture, especially at higher engine speeds and loads.
Leans the mixture: More air mixing with the fuel reduces the overall fuel density being discharged, leaning out the mixture.Reshapes the fuel curve: Air bleeds affect the shape of the fuel curve across the operating range, rather than just shifting the entire curve up or down like a main jet change. A larger main nozzle bleed will lean the mixture primarily in the upper RPM and high-load ranges.Delays system activation (to a degree): Larger bleeds can slightly weaken the vacuum signal in the fuel well, potentially delaying the point at which the main system fully activates or becoming more sensitive to engine load.
Pull-Over Enrichment System and Air Bleed Size
The Quadrajet's main metering system uses tapered metering rods that move in and out of the main jets based on engine vacuum (controlled by a power piston). The rods are pulled down into the jets by high vacuum (lean cruise mixture) and rise out of the jets under low vacuum/heavy load (rich power mixture).
More to follow, too much for one post.
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Bumping this to the top, as I am also curious about how these bleeds affect fuel curve.
Where's Tuner?
Continuation from above:
The "pull-over enrichment system" (also known as the power system) works with this mechanical/vacuum system, but it does not change the main nozzle air bleed size itself. The air bleed sizes are fixed (or manually tunable during a rebuild). The enrichment is achieved by:
Varying Jet/Rod relationship: The metering rods (which have different diameter steps) move up and down in the fixed-size main jets to change the fuel orifice area.Air Bleed Interaction: The pre-calibrated air bleeds then interact with the fuel flowing through this variable-area jet/rod system to maintain the desired emulsion and fuel curve at different loads and RPMs.
In short, the pull-over enrichment system uses the existing, fixed air bleed design in conjunction with variable fuel metering to achieve the required enrichment, rather than changing the size of the air bleeds themselves.
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Now, here's how I think this ties in with what I've been seeing on the AFR gage.
My carburetor is originally from a Cadillac, and it has the POE system, or rather it used to.
In an effort to make it like a Chevrolet carburetor by matching the specs, I did remove the tubes for the POE, thinking that it was affecting the results. Eliminate the POE, eliminate the variables, right? Uh, not so sure now.
The problem that I see now is that it's running extremely lean throughout the rpm range, so apparently the POE system was kicking in and supplying the extra fuel.
These larger main nozzle air bleeds are definitely affecting the AFR.
As I see it, I either need to reinstall the POE tubes and recalibrate with them in place, or start over with huge jets and try to get things under control with calibrations to the idle circuit and the main bleeds, since I see no way to change the size of the nozzle air bleeds. Unless you guys know of a method to do so?
I think I'll be practicing on some old junk Q-Jet cores to figure out if you can change the nozzle bleed size.
What a pain in the backside!!
I wonder if they make those set screws in a 4-40 or a 4-48 thread?
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Did you try raise the float like Novadude mentioned.
Here the tiny setscrews.
https://www.mcmaster.com/products/screws/set-screws-2~/thread-size~4-40/brass-cup-point-set-screws/thread-size~2-56/
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Did you try raise the float like Novadude mentioned.
I haven't done anything with it yet, I've been hijacked into doing stuff for the wife, but I think I'll have a chance to do something with it today.
Thanks for the link for the 4-40 set screws. I was actually able to find those on my own a bit after posting the question. You just have to know what to enter in the search box, but I'm glad to have a backup in case I can't figure it out.
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You said you only had a phone. I'd never find them on a phone myself. They have 2-56 setscrews too, how good are your eyes?
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Lol, I wear trifocals, what does that tell you?
I'd have to buy the drill bit and the tap for something that small. The other problem is just handling things that small. The arthritis doesn't let me do a lot of little stuff like I used to do.
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Here's one of the questions I asked online to find out about these nozzle air bleeds.
How does the main nozzle air bleed differ from the upper and lower main circuit air bleeds on a Rochester QuadraJet and what affect would a larger main nozzle air bleed have on the fuel curve and does having a pull-over enrichment system change the main nozzle air bleed size?
The previous posts are the resulting answers.
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You just about like me just had cataracts done in the last couple weeks & yea my hands don't work that great either.
I've read several stories about emulsion bleeds on Holleys, but not on Qjets.
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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.
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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.
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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.
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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.
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.
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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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I thought reducing the UIAB would help to pull a bit harder on the transfer slot. I know when I reduced the UIAB on mine it richened up a bit just off idle. I think I went .046 to .043.
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What is the number of the carb you are working with again?
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What is the number of the carb you are working with again?
It's a Cadillac carb #17057230.
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I thought reducing the UIAB would help to pull a bit harder on the transfer slot. I know when I reduced the UIAB on mine it richened up a bit just off idle. I think I went .046 to .043.
I observed the same. With colder fall temps before the engine was completely warm, it was a little too lean off-idle. I recently went from 0.043 to 0.040 and it made more of a difference than I would expect from such a small change. The air horn UIAB location seems to be sensitive to small changes. I would say it got 0.5 A/F ratio richer in the off-idle to 1500 rpm range. Maybe try going .002-.003 smaller on the upper idle air bleed before tearing into the carb.
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I haven't changed the UIAB size since I relocated them other than the initial reduction from .048" to .044" when I did the relocation.
Before I try anything else I'll swap out the .044"s for a pair of .042"s and see what that does to the lean zone. It would be nice if it really is that easy, but nothing ever works out that easy for me. Here's hoping my luck changes.
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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.
agree
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.
agree
Here is my theory (and I may be wrong)
1. The nozzle bleed sees the same pressure as the brass fuel nozzle.
2. The nozzle bleed is effectively out of the fuel path. It draws air through the MABs, but that air never mixes with the fuel going through the brass nozzle, since it's location is above where the fuel is flowing.
3. A larger nozzle bleed probably reduces the "pull" on the fuel through the main well, and may reduce the amount of MAB air mixing with the fuel coming out of the brass fuel nozzle. Basically it is an air leak that is at the same pressure as the brass nozzle in the venturi. This may delay the start of the main system?
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)
I do this on all my carbs (but mine have the bypass air coming out into the bores). I open the existing bore bypass air holes to ~.106 or so, and install 1/4-20 brass screws in the vertical bypass passages in the baseplate at the baseplate-to-main body gasket surface (setscrews thread in from the top). This just moves the bypass restriction further up the passage. This way I can drill the setscrews to whatever size I want.
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" The nozzle bleed is effectively out of the fuel path. It draws air through the MABs, but that air never mixes with the fuel going through the brass nozzle, since it's location is above where the fuel is flowing."
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I agree with this except that I believe that the nozzle bleed air "difuses" the mixture further as it's flowing out of the nozzle itself, flowing around the nozzle to mix below it, effectively leaning it more than what a smaller nozzle bleed size would, such as a Chevrolet .029" bleed size.
On this Cadillac carb it has/had a POE system which filled in where the main system fell short, and i saw the effects of that after removing the tubes, it was lean from top to bottom once out of the idle circuit.
To offset that, raising the float basically makes it easier for the fuel to be pulled through the primaries. By increasing the jet size instead of raising the float when I was on my way home helped a bit, but didn't fix the problem. The idle circuit was supplying enough fuel for its intended range, but once beyond this range it leaned out.
The bigger nozzle bleeds really mess up the standard setup, the normal "rules" don't apply here.
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"I open the existing bore bypass air holes to ~.106 or so, and install 1/4-20 brass screws in the vertical bypass passages in the baseplate at the baseplate-to-main body gasket surface (setscrews thread in from the top). This just moves the bypass restriction further up the passage. This way I can drill the setscrews to whatever size I want."
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That's a much better idea than mine, I like that. I kept wondering how to change the "direct" primary opening for the bypass air without the risk of something dropping into the engine if it comes out if you had to use a carb with this style of base plate. This solves the problem nicely. Thanks for sharing your solution.
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"A larger nozzle bleed probably reduces the "pull" on the fuel through the main well, and may reduce the amount of MAB air mixing with the fuel coming out of the brass fuel nozzle. Basically it is an air leak that is at the same pressure as the brass nozzle in the venturi. This may delay the start of the main system?"
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This makes sense on both counts, you may be on to something here.
In order to offset the effects of the bigger nozzle bleeds you would have to use a bigger jet, but that would change the AFR throughout the whole RPM range, not just for the main circuit, your idle would be richer too. But you could argue that the idle mixture screws can be turned in to fix this.
Your transition slots and the LIAB's should still fill in from idle to main systems, right? But how do you tune the transition if necessary?
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I forgot to mention that when I was checking the venturi for nozzle drip that I also was using a tach/advance timing light to see at what speed the mains started to draw. In drive with the wheels blocked I ran it up and noticed that it began to draw at around 1000 rpm. Now this was after raising the float level. If memory serves, when I checked this earlier it would start to draw about 1400-1500 rpm, but I didn't write anything down then so I'm not positive on the rpms, but I do remember being surprised when it started at 1000 this time.
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In thinking about this nozzle air bleed size, it looks like it's related to the cubic inch size of the engine. Here's my thoughts on this.
A 283ci engine running at 500rpm is moving 40.94cfm at 100% volumetric efficiency.
A 500ci engine running at 500rpm is moving 72.33cfm at 100% volumetric efficiency.
(Of course 100% volumetric efficiency is not possible, I just used this for quick/easy reference).
If both engines are drawing through the same size Quadrajet and running at the same rpm, then the actual speed of this volume of air flowing through the carburetor is quite different, and would require the main system to react differently.
If the 283 begins to draw from the main system at say 1000rpm just for arguments sake, it's moving 81.88cfm to get the mains working at 1000rpm.
Now take this exact same carburetor and bolt it onto a 500ci engine. In order for this engine to move 81.88cfm of air it would have to run at 566rpm. Much too slow to be pulling fuel from the main circuit, right? But if it's the same carburetor that you just had on the 283, the 500ci engine basically only has to idle in order to move this same volume of air, right?
So, how to raise the speed at which the main circuit begins to operate.
The jets won't change this, they would change the AFR though.
Upper Main Air Bleeds, these would have an effect, but what exactly? The AFR?
Lower Main Air Bleeds, not sure exactly what effect they have on the start of the main circuit.
Main Nozzle Air Bleeds, these apparently do have an effect on when the main system starts, but why?
Apparently the bigger the nozzle bleed is, the later the main system begins to work, the bigger the nozzle bleed is, the more air it takes to get the main system to work.
If someone has access to the specs for multiple carburetors on different sized engines, can you compare the nozzle bleed sizes and see if it correlates?
For instance, a '67 283 Chevy, '69 327 Chevy, '77 350 Buick, '77 350 Chevy, '77 350 Olds, '77 350 Pontiac, '72 396/402 Chevy, '77 425 Cadillac, '69 427 Chevy, '69 428 Pontiac, '67 429 Cadillac, '77 454 Chevy, '76 455 Buick, '76 455 Olds, '76 455 Pontiac, '72 472 Cadillac, and a '76 500 Cadillac for a wide range of engines both large and small from a narrow range of years from all GM brands to see if the engine size determines the nozzle bleed size, or if maybe it's the Cadillac engine design that makes it need a different setup.
I think the size of the engine is a contributing factor in the nozzle bleed size. This is the only reason I can think of that would explain the size difference.
Rick
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Quadrajet was never used on any Chevy 283 engine, every Chevy Quadrajet ever made uses .029" nozzle bleeds.
All 1968 and later Buick Q-jets uses .054" nozzle bleeds. All earlier uses .029" nozzle bleeds.
All 1975 and later Cadillac Q-jets uses .054" nozzle bleeds. All earlier uses .029" nozzle bleeds.
All 1975 and later Olds Q-jets uses .054" nozzle bleeds. All earlier uses .029" nozzle bleeds.
All 1975 and later Pontiac Q.jets uses .054" nozzle bleeds. All earlier uses .047"-.050" nozzle bleeds.
My conclusion is that the larger nozzle bleeds are more of an emission feature, that can be cured by several methods, like float level and modifications in the idle/lowspeed circuit.
Right or wrong, I have never factored nozzle bleeds into the equation when I have modified/adapted the calibration for a particular engine other than the original.
I rarely if ever change the main jetting or the air bleeds on the primary side, but do most of the work in the idle/low speed circuit idle tubes, downchannels, off-idle holes and the holes for the mixture screw tips.
If I am using a carburetor that has the extra pull-over tubes in the primary circuit, I remove them and look at the main jetting in the nearest model of the same brand without them and change to this jetting if possible, depending on the size of the main air bleeds.
Then it is the "tip-in" test and the test run that determines whether I have succeeded.
Based on this, I have accumulated a considerable amount of recipes, for decades, that are useful for different scenarios.
HTH some.....
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"Quadrajet was never used on any Chevy 283 engine."
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I wasn't sure on this one, but thought I'd throw it in simply because of it's size for comparison.
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"Every Chevy Quadrajet ever made uses .029" nozzle bleeds."
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So engine size, (and apparently emissions), never factored in with Chevrolet, interesting. Unless it's because the Chevrolet engines were more efficient than the other GM lines were. The Buick engine was the first to be discontinued, the last one used in '76, followed by Pontiac's 301 I think in '81, and then the Oldsmobile 307 in '89 in the Cadillac Brougham.
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"All 1968 and later Buick Q-jets uses .054" nozzle bleeds. All earlier uses .029" nozzle bleeds."
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So Buick started this. In '68 I wouldn't think that they were too worried about emissions. Engine size didn't factor in here at all, also interesting.
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"All 1975 and later Cadillac Q-jets uses .054" nozzle bleeds. All earlier uses .029" nozzle bleeds.
All 1975 and later Olds Q-jets uses .054" nozzle bleeds. All earlier uses .029" nozzle bleeds.
All 1975 and later Pontiac Q.jets uses .054" nozzle bleeds. All earlier uses .047"-.050" nozzle bleeds."
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It's interesting to note that all the rest of GM didn't start using the larger .054" nozzle air bleeds until 1975, but Pontiac was using only slightly smaller ones before '75.
I hadn't ever given them any thought until I began modifications on this 77 Cadillac carb.
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"If I am using a carburetor that has the extra pull-over tubes in the primary circuit, I remove them and look at the main jetting in the nearest model of the same brand without them and change to this jetting if possible, depending on the size of the main air bleeds."
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When I tried to "copy" a Chevrolet spec with this one I ran into all sorts of issues, which I attribute to the bigger nozzle bleed size, and the removal of the POE tubes.
And yes, a jet size increase helped a bit, and the higher float level improved it somewhat. I haven't gotten around to trying different idle system changes yet.
Today I was making a list of the various drilled set screw sizes that I might need for the different bleeds and restrictions before I start working on it again. I want to have an assortment of them ready so it'll be quicker to make the necessary changes.
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My conclusion is that the larger nozzle bleeds are more of an emission feature, that can be cured by several methods, like float level and modifications in the idle/lowspeed circuit.
Right or wrong, I have never factored nozzle bleeds into the equation when I have modified/adapted the calibration for a particular engine other than the original.
I rarely if ever change the main jetting or the air bleeds on the primary side, but do most of the work in the idle/low speed circuit idle tubes, downchannels, off-idle holes and the holes for the mixture screw tips.
This makes sense to me. I believe that the Nozzle bleeds will delay the start of the main circuit, so you can probably fill that "hole" with idle circuit modifications to get more fuel out of the idle circuit in that off-idle transition range.
What puzzles me about this particular case, is that it seems like the Cadman-iac's various trials with IABs and IDCRs should have been putting plenty of fuel to the transfer slots, yet the problem persists.
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This makes sense to me. I believe that the Nozzle bleeds will delay the start of the main circuit, so you can probably fill that "hole" with idle circuit modifications to get more fuel out of the idle circuit in that off-idle transition range.
On this subject Novadude, if you increase the UIAB's this will lean out your idle mixture, if im understanding it correctly.
By increasing the LIAB's then this will fatten up, "enrich" the off-idle mixture, again if I'm understanding it correctly.
Enlarging the idle tubes enriches the whole range of the idle circuit, correct?
And by increasing the IDCR's, this would have the same effect as increasing the idle tubes, but also increases the effective range of the idle circuit, is that correct?
If not, what am I missing here?
And increasing the idle by-pass air it allows you to close the throttle plate more to prevent any nozzle drip, in addition to being able to increase the idle speed without changing the throttle position, I think, right?
I was up late last night reading Doug Roe's books trying to get it straight in my mind.
Another question, going through what I need for various sized drilled set screws I couldn't remember where the 10-32 screws were used, or did I order those by mistake? Do you use this size for anything?
Kenth, Novadude, thanks for all the help and patience on this. Im beginning to think that I don't understand everything I know about this carburetor, lol!
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I used the 10-32 to make idle tubes, you need some extra tubes. If you have a couple of junk cores to pull the tubes out of you're set. I can't remember off hand the length of the screws 1/2 in. I think & the tube have to be shortened the depth of the hex in the screw.
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I used the 10-32 to make idle tubes, you need some extra tubes. If you have a couple of junk cores to pull the tubes out of you're set. I can't remember off hand the length of the screws 1/2 in. I think & the tube have to be shortened the depth of the hex in the screw.
Ok, great, thanks. I tapped the holes in a junk core just to practice, but I was using the 8-32 screws. Do you have to go to the 10-32 because of the diameter of the tubes so you don't lose the hex portion, or was there another reason?
By the way, those look great! Nice solder job!
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:-*On the primary air bleeds in the air horn, do you go with the 10-32, or a 1/4-20 set screw?
Just practicing on a core, i noticed that you can use either one, it just depends on how big you want the drilled holes in the set screws.
But it's easy to drill a small hole in a big screw, much harder to drill a big hole in a little screw, your hex drive disappears.
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Only time I ever tried different MAB sizes was one a carb removed the outer booster to try it & I mess with the bleeds but never got it to work. But I think I used 8-32 I think the h.ex is 5/64.
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Only time I ever tried different MAB sizes was one a carb removed the outer booster to try it & I mess with the bleeds but never got it to work. But I think I used 8-32 I think the h.ex is 5/64.
Thanks Cruiser, I put some 8-32 screws in it. Those are big enough for up to an .080" hole in think.
Rick
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On this subject Novadude, if you increase the UIAB's this will lean out your idle mixture, if im understanding it correctly.
Yes. I believe that is true.
By increasing the LIAB's then this will fatten up, "enrich" the off-idle mixture, again if I'm understanding it correctly.
I'm not sure about this one. I think larger LIABs will actually lean it out until the throttle blades get past the LIAB. By this point, the main system will already be active. In my experience, the larger 0.078" LIAB carbs never seemed to run as well off-idle as the ~0.062 LIAB carbs.
Enlarging the idle tubes enriches the whole range of the idle circuit, correct?
I think this is true
And by increasing the IDCR's, this would have the same effect as increasing the idle tubes, but also increases the effective range of the idle circuit, is that correct?
If not, what am I missing here?
Larger IDCR will increase the amount of mixture getting to the transfer slot and idle screws. If the UIAB is too large, or idle tube is too small, it could still end up lean off idle. At least I think that is how it works.
And increasing the idle by-pass air it allows you to close the throttle plate more to prevent any nozzle drip, in addition to being able to increase the idle speed without changing the throttle position, I think, right?[/quote']
Agree
I was up late last night reading Doug Roe's books trying to get it straight in my mind.
Another question, going through what I need for various sized drilled set screws I couldn't remember where the 10-32 screws were used, or did I order those by mistake? Do you use this size for anything?
Kenth, Novadude, thanks for all the help and patience on this. Im beginning to think that I don't understand everything I know about this carburetor, lol!
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I used the 10-32 to make idle tubes, you need some extra tubes. If you have a couple of junk cores to pull the tubes out of you're set. I can't remember off hand the length of the screws 1/2 in. I think & the tube have to be shortened the depth of the hex in the screw.
OK I see I put 1/2 inch should have been 1/4 inch. I'll measure later to make sure.
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Allstate carburetor sells pre-made threaded tubes, but tap size is weird (uncommon). I've used these before.
https://allcarbs.com/product/rochester-quadrajet-idle-tubes-threaded/?v=0b3b97fa6688
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I've seen them too, after I had done mine. Wonder why that thread, so they could sell a tap too?
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No idea. Someone on the other carb forum (when it was still active) bought a quantity of taps in that size and sold me one for a good price. They are pretty expensive to buy just one. I see Allstate sells them in a kit with a tap, but they are not cheap.
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OK I see I put 1/2 inch should have been 1/4 inch. I'll measure later to make sure.
Well I can tell you that the 1/8" long ones are definitely too short for this. A 1/4" long one would probably be perfect.
I made a couple last night but wasn't happy with them, I'm afraid that the tubes will come out if subjected to too much heat or vibration, even though they're soldered in.
Novadude,
Thanks for the explanation/clarification on the idle circuit. I appreciate it.
Rick
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I measured them & I used 1/4 inch for the tubes.
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No idea. Someone on the other carb forum (when it was still active) bought a quantity of taps in that size and sold me one for a good price. They are pretty expensive to buy just one. I see Allstate sells them in a kit with a tap, but they are not cheap.
https://www.mcmaster.com/products/taps/taps-1~~/thread-size~10-40/chamfer-type~plug/chamfer-type~taper/