Relocating the UIAB, are size adjustments required?

[Cadman-iac]

  Just a quick question, i just relocated the upper idle air bleeds on my carburetor, but I was wondering, because of the new location and the different route that the air takes to get to the original bleed location, do you need to resize for any reason?

 The air now runs through the air horn to the gasket where it needs to do a small zigzag in order to continue on down to the channel restriction, whereas before the air entered just above the restriction.
 However, because of the new location (on the top of the horn basically), there should be less of a flow hindrance as the air is coming at it straight-on, a sort of ram effect so to speak. The original bleed location being next to the venturi and aiming slightly downward, does not get any "ram effect".
 So do the two issues cancel each other out and there's nothing to worry about, or are these something to be concerned about? Just curious.

 Rick

[Cadman-iac]

Here's a couple of pictures showing the new location of the upper idle air bleeds and the primary air bleeds in the air horn.

And a couple more showing the plugged original UIAB.

[novadude]

The air horn UIAB will typically need to be smaller than the main body, indirect UIAB.  On my current carb, I am running a 0.040 UIAB (air horn with screw in bleed like yours), 0.062 LIAB, 0.029 idle tube, and 0.046" IDCR.  Plenty of fuel for a nice idle with a 217/225 @ .050, 108 LSA cam.

[Cadman-iac]

The air horn UIAB will typically need to be smaller than the main body, indirect UIAB.  On my current carb, I am running a 0.040 UIAB (air horn with screw in bleed like yours), 0.062 LIAB, 0.029 idle tube, and 0.046" IDCR.  Plenty of fuel for a nice idle with a 217/225 @ .050, 108 LSA cam.

 Right now I've got the UIAB's set at .048 in the new location on the air horn.
 I also reduced the LIAB's to .061 from the stock size of  .078.
 I changed the IDCR's back to .046, which is the original size.
 The idle tubes are now at .040, but are still larger than stock which was .035.

 I'm running a Comp Cams HR with 206/210 @ .050, with an LSA of 112, so it's got a really smooth idle and pulls well, much better than the original flat tappet TBI cam did.

 If I reduce the UIAB more, just how far should I go, .044?
  Should I change the idle tubes back to the original size  .035, or am I ok with where I'm at now?

  Rick

[Kenth]

What engine?

[Cadman-iac]

What engine?

 Sorry,  it's an 88 Chevrolet 350 with the stock swirl port heads, I think they're number 193's, have to check again to be sure.  Also has 4 relief flat-top pistons and the thicker composite gaskets, so compression is about 9.0:1 maybe a bit less, not positive.

 The carb also has the larger  .055 main nozzle air bleeds, and i have no idea how this plays into the combination. It's a 1977 Cadillac 425 carb #17057230.

 I've been trying to set it up like a typical SBC carb, but the nozzle bleed size changes the equation, unfortunately I don't know in what ways.
 Is this something to worry about or just ignore it?

[novadude]

That sounds like a LOT of idle fuel for a stock-ish SBC.  Are you saying it is still lean at low RPM part throttle?  See my idle calibration above... this is a 9.6:1 355 vortec head SBC with a 270/278, 217/225, 108 LSA cam.  Light throttle, low RPM A/F is in the 14s with no lean surging, etc.  My idle tubes are much smaller than yours.

I wonder if you are on to something with the theory of .055 MAB at nozzle significantly delaying the start of the main circuit?  My carb is a 17080204, so it has the Chevrolet 0.028 bleed in this location.

[77cruiser]

I had a carb several years ago that was acting up at light throttle & found that I had put the wrong base to main body gasket in it.

[Cadman-iac]

That sounds like a LOT of idle fuel for a stock-ish SBC.

 Yeah, going by the AFR gage as I'm opening the throttle, just over idle up to around 900/1000 rpm, it goes lean, as much as 17.5-18.0, and I'm afraid that is too lean.  If you hold it steady when this happens it'll stay in that range.

 I went through it today and changed the UIAB's to .044" and the idle tubes to  .032".

 Here's the setup as of right now.
 Idle tubes................. .032", stock is 035
Chnl restrictions....... .046", stock is 046
UIAB, relocated......... .044', stock is 048
LIAB.......................... .061", stock is .078
Bypass air................ .045", stock is  .039
Idle discharge......... .088", stock is  .052
Float.........................13/32, stock is 13/32
PAB-Body................. .040", stock is  .040
PAB-Horn................. .070",  stock is  .070
Main noz. bleed. .......055", stock is  .055
Main jet..................... #72,   stock is #72
Primary rod.........#44J, Cliff's   stock is #52P
PP spring.............3-10" blue, stock 3-10"
APT set.................3 turns,  stock 2 ¹/⁴
Ac pump noz........... .025", stock is  .025"
Sec. Rods....................BA,  stock is  DS
Hanger.........................K,    stock is  T


 I haven't driven it yet, but I've got an appointment tomorrow morning in Tucson, so it'll get a 240 mile run. I'll get to see how well it does with this newest setup. Hopefully it will be better.

 I've checked and double checked the base to bowl gasket, it's correct.

 I want to thank you guys for helping me try to figure out where I went wrong. I really appreciate the help Novadude, 77cruiser, Kenth, thank you.
 I'll let you know what happened when I get home.

 Rick

[novadude]

That looks like it should work great in a mild 350.  Report back and let us know the results.  It seems odd to me that you were having issues with the old combo, as that should have been plenty of idle fuel with the 0.048 main body IAB and .040 tubes.  I wonder if you are getting some kind of air leak somewhere in the idle circuit? 

[77cruiser]

Hope it works out.

[Cadman-iac]

  Well,  im in Tucson now, it was a mostly downhill run, from 4600 to 2600+/-.
 When i started out I was really panicking as the AFR's were going off the charts, 18.0 plus right from the opening of the throttle and on up to 3300 rpm. If I goosed it then the afr would drop down to 15.0 up to 16.5 depending on the load, but a light load and it's off the charts. I went about 50 miles while watching the AFR gage and the 2 temp gages to be sure it wasn't getting too hot. At this point I stopped and raised the APT a full turn to see if it would make any difference, which it slightly improved, now it's running at 17.0 at 10" vacuum at 3300 rpm.
 I went another 50 miles before I raised the APT another half turn, but that didn't make any difference.
  Before I head home im I'm going to change the idle tubes to a .035 and see where that puts it. If this doesn't improve anything then I'll increase the UIAB's to a .046".
 I have my drill and bits and the new set screws to make bigger LIAB orifices if necessary, so I can go up beyond the  .061 that's in there now. I definitely didn't expect the results I got with this last setup. I think I may have just gone too far in the other direction with this setup.

 As for an air leak in the idle circuit, I don't see anything that would be a source, everything looks good, gaskets and parts.

[novadude]

10" of vacuum at 3300 rpm... seems like at that point, the carb would be almost entirely on the main circuit.  I'd try cranking up the APT another full turn.  I'm thinking it needs smaller rods or larger jets.

Bigger IABs will just lean it out even more at light load when on the idle and transition circuits.  I would not go bigger on these.

AFR meter aside, how does it drive?  I would think at 17.0 you'd have some lean surge. If it feels good, maybe the AFR meter is the problem?  Collector leak or somthing upstream of the sensor?

My current set-up goes down the road at 70 mph (~3300 rpm) at 15.3-15.5 AFR.  If I increase the load by dipping into the throttle to pass, it can swing up to high-16s before the power system kicks in and it goes rich.  IMO, this is totally normal / desirable.  It seems like 17:1 AFR at steady cruise would likely go leaner as you dip into the throttle to pull hills, etc.  Leaner than 17:1, I would think the engine would definitely object. 

[Cadman-iac]

 Yeah, I thought about it before I left Tucson, I raised the APT another half turn, but little effect.
 Halfway home I changed the jets to a #73, this was a step in the right direction. The AFR dropped to 14.5 to 16.0 if I remember correctly. I need to use a voice recorder to make notes as it's happening. There's too much information to try to remember it all,  at least for my feeble mind anyway, lol!

 Thanks for your expertise and input. You have confirmed what I had been thinking, I appreciate it.
  Now I just need to fine tune it now that I know what to fine tune. At least I've been keeping good records on what I've done to it as far as the modifications. The recorder will help with getting the results correct.

[77cruiser]

How does it run? Does it surge or buck at all. Do you have any exhaust leaks?