"Maybe you'd get a little more power with a richer mix, but some people are OK with near stoich at idle. Leaner saves fuel and if no misfires, less HC and CO out the exhaust. Only downsides are a little more NOx and the engine may be a little hotter than with a richer mix. "

The main reason I keep fiddling on idle adjustment is that it sounds a bit rough and also it seems to me the engine is picking up heat when idling for a long time. Last year I was in stop-and-go traffic at the entrance of a swap meet for more than 30 minutes. Water and oil temperatures went up quite a bit, although the engine didn't show any running problems. No fuel boiling, rougher than usual idle, water boiling or such. But the gauges made me a bit nervous...

Fair enough. Keep working on it then. I just didn't want to tell you to keep tweaking if you were satisfied.

When catalytic converters were first added, they controlled the NOx. The high NOx at idle was in part due to the leaner idle mixes introduced to bring down the CO and HC. Idle speeds were increased, and idle stop solenoids solved the run on from hot chamber and wider idle throttle opening. My understanding is that with controlled EFI, idle mixes close to stoich continued to be the norm.

Here's a post by Tuner with graph plotting Combustion products vs. AFR. As he explains, even on stock engines idle mixes tended to be much richer than stoich until emmissions concerns started pushing them leaner.

The other thing that will make an engine a little hotter is retarded timing. On my fully smogged '85 AMC 360, there is a coolant temperature switch to provide the distributor's vacuum advance full manifold vacuum if coolant hit 220 degrees F. Cold, the vacuum advance gets a mix of ported and manifold vacuum. At normal operating temperatures, it gets ported only.
So this shows that timing was signficant enough effect on coolant temperature that the OEMs used it when emmissions and drivability got really tricky to balance.

Back to idle fuel mixes. The research from performance oriented (non-emmissions) testing appears to favor richer than stoich at idle. See
the AFR vs. Load graphs published by Larew,Taylor and Obert. They're posted in this thread AFR Characteristics vs. Load. It is worth a looking at even though the thread got a little jumbled when the threads were merged.

This is what mine usually end up at w/a good ign. curve OR locked out.......1 1/2-2 turns or more out w/a pcv valve and I gave up trying to tune a carb to EFI afr's and w/my 276-281 @ .050, never gonna happen or you will get lean surges leaving a stop light or at tip in before cruising. I use the w/b as a "directional tuning tool" and that's it and once I tuned w/one, my junk went from low 10's right into the 9's.........just like that.............

In addition to all of the other good info, I would focus more on how the car idles, feels and drives off idle than the afr #`s.........Shooting for this 14 to whatever "magic number" doesn't work in a lot of cases w/my street/strip testing and customers feedback even on Drag Week where they DRIVE their stuff...............

Did some testing today. Tried lots of things that you guys suggested, thanks a lot for that!
PCV:
Pulled the hose from the carb and blocked the fitting: No influence.
Brake booster:
Pulled the hose from the intake fitting and blocked it. Same, no influence on AFR.
Measured fuel pressure, Mr. Gasket gauge:
Needle is swinging from 4.5 to 7" at idle, with the average slightly below 6". Instrument seems to be not damped good enough. But I assume nothing is wrong with my fuel pump or pressure. No fuel dribbling out of the boosters. Fuel level stable at middle of glasses (pri and sec).
Closing the IABs:
Getting these blocked with my fingers isn't an easy task. The choke plate is in the way. And the IABs have screwdriver slots where they can still pull some air with the finger on top. But: When I put my fingers on them the rpm goes down a bit. Not much, but you can hear it.
Turning in the idle mixture screws:
My setting was 1 turn out on each of the four screws. Closing one pair (doesn't seem to matter which one, pri or sec) half a turn would make the engine slightly rev up. Turning in the second pair of screws brings revs down again. Screwing them further into the seat (one pair, the other is still half turn out) would make the engine run very rough. Closing all screws kills the engine.

I left it at 1 turn each and did some part throttle driving. Once I am rolling the AFR goes to the rich side, AFR around 13.0. I found out that fiddling with secondary throttle angle and the turns of the mixture screws has great impact on part throttle AFR. Opening the sec blades makes it run leaner overall because of the additional air around the blades. And while the mixture screws don't influence my idle AFR they do very much influence part throttle AFR. Indeed they are very touchy. I found a good setting of blades and mixture screws with my current IFRs and IABs. Car drives very well and doesn't seem to have flat spots. No off idle bog.

I am running out of ideas. Checked a lot of stuff, all seems to be as it should. Agree?

Something doesn't seem right.

a. Engine not effected by blocking PCV. It should have dropped or increased in rpm and vac.

b. Fuel pressure swinging at idle. That seems like a lot of swing for idling. See what others say. Its been a few years since I've had a gage on - my memory is fuzzy on this.

c. The idle mix screws having a large effect on off-idle driving. They should have a large effect on idle and small effect on off-idle. Percentage wise, the fuel should be coming through the least restrictive path - the transfer slots.




This is interesting. Doesn't want richer.


Wants a little leaner - at least with no load on it.


So they do seem to be working.

On a cam like that, vacuum goes up when opening throttle from closed. The greater pressure difference, the more flow, up to around 14"Hg when it chokes.

If you got it where you like it, then that's good.

it sounds like your AFR gauge ,sensor is not working properly.?

is there an exhaust system on the car or open header ?

The car has Hooker 1 7/8" headers and full 2.5" exhaust system. Sensor prong is welded into the collector. I thought about leaky headers. The effect would be that the gauge would show too lean, but in reality runs richer than shown. But the engine should respond to turning the mixture screws nonetheless, for example by cleaning up the idle quality or changing rpms. That should both be heard, even if I didn't have a gauge. But not much can be heard, therefore I assume the sensor shows what really is happening.

PCV: I did several tests, maybe I mixed things up. Will try this again. What I definitely remember is, when I pull off the PCV or brake booster hose and leave the fittings open, rpm picks up a lot because of the big air leak and leans out to at least AFR 16. I'm pretty sure that idle AFR stayed in my usual range of 14.5 when I pull off either of that hoses and block the fitting. Can't remember if rpms changed when pulling off and blocking PCV. I agree, the PCV adds a certain amount of air to the mixture which is not there when blocked. So AFR should change. But it doesn't. PCV is new and clicks when shaking it.

"On a cam like that, vacuum goes up when opening throttle from closed. The greater pressure difference, the more flow, up to around 14"Hg when it chokes."
Sorry, don't understand that. What chokes?

Revving up with additional air in neutral is normal.

There are other reasons a wideband may give misleading AFR besides the program or electrics. Anything that results in a different amount of O2 in the exhaust can fool it. Misfires, or burns that don't combust and progress completely at the temperatures expected. I'll post some links later.
It's very good that you are paying attention to rpm, vac and how it sounds.



[edit: corrected]
I got that backwards. The air mass moving past the throttle plates is choked at high vacuum. So when on the idle circuit at any given throttle position with manifold vacuum over 14.8 psi, the amount of air will be limited.
At 0.528 absolute pressure difference, the mass flow of the air can not be pushed any faster.

Whenever the manifold vacuum is below 14.8", changing the manifold vacuum will result in a change in mass flow of the air. This makes the idle to off-idle a little trickier with a hot cam because than with a stock engine that idles with high vacuum.

Below is a snip from Bruce "Shrinker" Robertson
It's a little bit of an aside but maybe you'll find it interesting.