What are these holes for?

[Cadman-iac]

Quote
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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.

Quote
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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.

 Quote
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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.

   Quote
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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.

  Quote
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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.

[novadude]

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. 

[Cadman-iac]

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!

[77cruiser]

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.

[Cadman-iac]

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!

[Cadman-iac]

 :-*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.

[77cruiser]

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.

[Cadman-iac]

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

[novadude]

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!

[77cruiser]

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.

[novadude]

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

[77cruiser]

I've seen them too, after I had done mine. Wonder why that thread, so they could sell a tap too?

[novadude]

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.

[Cadman-iac]

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

[77cruiser]

I measured them & I used 1/4 inch for the tubes.