Can't get idle down. Several cases...

[Marx3]

I have built 3 carbs for Cadillac 500 engines recently and used the same recipe for all 3, but with varying results, for some reason. 1 came out excellent and the other 2 did too, except I couldn't kill them by backing out the idle screw. These 2 wanted to idle at 700-800 at least ( which is a bit too much for a Cadillac if you ask me )

The question is, was I lucky with the first one meaning the recipe is not fit for a stock 500, or must there have been something wrong with the other 2?

All 3 engines were stock smog 1975-1976 Cadillac 500 cui's in stock Deville /Eldorado.
 Here are the recipe's
1 engine ( the one that worked perfectly and idles as low as I wanted it to.
.038 tubes
.059 DCR
IAB's .070
Bypass ~.100
The 2 engines that would not idle low were set up like this
.037 tubes
.059 DCR
Upp. IAB .065
Low. IAB .085
Bypass ~.100

Tubes .037
DCR .051
Upp IAB .065
Low IAB .085
Bypass ~.050

Is this setup simply just too generous for a 500? On the 2 latter engines I could back the idle stop out without killing the engine.
All 3 engine ran perfectly

[Frank400]

   I'm thinking that the carb that does not want to idle low is getting air from somewhere else, maybe the throttle blades not centered in the bores, or the secondaries staying open a bit ? 

   Don't know what carb numbers you're using but if the problematic one is one with a hot air choke with the passage left unplugged ?

[Marx3]

Cant remember the ID numbers for all three carb, but they where all rebuild with new shaft bushings, centered plates and all engines where meticulously checked for vacum leaks.
I don't think that a hot air choke housing left unplugged would let the engine get air enough to make the engine idle so high?
I feel there is a pattern here and that's why I thought maybe the recipe was too generous for a stock engine?
A stock SBC with these idle specs would propably have too much idle fuel and bypass air to be able to idle below 700-800 rpm, but would that go for a giant 500 too?

[77cruiser]

 It has to be getting air from someplace. vac. leak, throttle shaft, or throttle plates. I the IFR was too big it would be rich is all.

[Frank400]

can you lower the idle speed by turning the idle mixture screws in ?  Some carbs have really long transfer slots which show quite a bit under the throttle blade.  This could be a problem (I'm guessing) if the idle recipe is a bit on the rich side. 

[Marx3]

Yes I this was the only way to lower the idle below 7-800.
Not in any way ideal, I know.
So I guess the recipes ARE too generous.
The weird things is, one carb was a early 80's Chevy unit, one was the original 1976 Cadillac unit and one was an oddball 7045186. That one was definitely some sort of service replacement or rebuild unit.
It had main air bleeds about 4-6 mm ( 5/32"-15/64" ) long, making them stick out of the air horn and Venturi.
My point is, that these were 3 completely different carbs, set up the same way on the same engine. And 2 of them had the exact same problem.

[Cliff Ruggles]

Too much timing on a basically stock engine can do the same thing.  If you are running a lot of timing and manifold vacuum to the advance on a big 500 Caddy engine you could loose control of the idle speed as it would be sucking pretty hard on the entire intake system. 

The engine still has to get air from someplace, and if all the throttle plates are tightly shut at idle, then bypass air is all that is left, aside from the little bit of air it gets from the PCV system.....Cliff

[Marx3]

The first engine was at ~18 degrees initial with ported vacum if I recall correctly, and that did not give me any trouble keeping idle lower than 800.
The two other engines had 15 degrees initial, but can't remember if VA was manifold or ported.
I see your point though. I will check if it helps to put the VA on a ported source.

Actually I am having the same problem on my stock SBC.
The carb is calibrated for the 214/224@050 cam I am about to install ( along with 10:1 cr ).
.036 tubes
.051 DCR
.070 IAB
.085 mixture holes
.065 bypass.
Going to ported VA helped get the idle down to 700 rpm with 17 degrees initial.