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]

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.

[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.

[77cruiser]

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.

[77cruiser]

What is the number of the carb you are working with again?

[Cadman-iac]

What is the number of the carb you are working with again?

 It's  a Cadillac carb #17057230.

[novadude]

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.

[Cadman-iac]

  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.

[novadude]

 

 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.

[Cadman-iac]

  Quote
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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.

 Quote
‐------------------
  "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.

Quote
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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?

[Cadman-iac]

  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.

[Cadman-iac]

  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
 

[Kenth]

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.....