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Quadrajet Carb Talk and Tips / Re: What are these holes for?
« Last post by Cadman-iac on Today at 09:19:27 PM »
  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 Carb Talk and Tips / I need recommendations for quality drill bit set for Q-jet work.
« Last post by Mike Maner on November 04, 2025, 12:50:07 PM »
If anyone has any suggestions I would appreciate hearing from you. Thanks
Mike
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Quadrajet Carb Talk and Tips / Re: What are these holes for?
« Last post by Cadman-iac on November 04, 2025, 08:59:20 AM »
  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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Quadrajet Carb Talk and Tips / Re: What are these holes for?
« Last post by Cadman-iac on November 04, 2025, 07:20:09 AM »
  Quote
------------------
" 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."
-----------------

  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."
------------------

 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
---------------
  "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?"
----------------

  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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Quadrajet Carb Talk and Tips / Re: What are these holes for?
« Last post by novadude on November 04, 2025, 05:52:10 AM »
Quote from: Cadman-iac on November 03, 2025, 06:51:25 PM
 

 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

Quote
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?

Quote

 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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Quadrajet Carb Talk and Tips / Re: What are these holes for?
« Last post by Cadman-iac on November 04, 2025, 05:30:05 AM »
  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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Quadrajet Carb Talk and Tips / Re: What are these holes for?
« Last post by novadude on November 04, 2025, 05:02:10 AM »
Quote from: 77cruiser on November 03, 2025, 06:56:46 PM
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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Quadrajet Carb Talk and Tips / Re: What are these holes for?
« Last post by Cadman-iac on November 03, 2025, 07:24:12 PM »
Quote from: 77cruiser on November 03, 2025, 07:00:43 PM
What is the number of the carb you are working with again?

 It's  a Cadillac carb #17057230.
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Quadrajet Carb Talk and Tips / Re: What are these holes for?
« Last post by 77cruiser on November 03, 2025, 07:00:43 PM »
What is the number of the carb you are working with again?
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Quadrajet Carb Talk and Tips / Re: What are these holes for?
« Last post by 77cruiser on November 03, 2025, 06:56:46 PM »
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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