Tuning with an A/F gauge

[travis]

This is my first experience with one of these things, so please bear with me. I’m probably going to have a lot of questions.

How much fluctuation should you see in A/F ratio at a steady speed, whether it is idle or cruise? I’m seeing roughly a .8 to 1 range fluctuations at idle or any steady speed. This seems to be way too much...I’m wondering if I might have a slight misfire. Could this cause this much fluctuation? There are no vacuum leaks or exhaust leaks.

Can you tune the idle mix screws with just the a/f gauge? I was kind of surprised that even a 1/8 turn on the mixture screws can make such a significant difference in the idle a/f ratio.

Btw...this is with a brand new Edelbrock 650 AVS2 carb with annular boosters that I actually bought for another project. Drivability is dramatically better than that POS 670 Holley I had on before, but the Holley most definitely made more WOT power. I’m not impressed...at least not yet.

[cardo0]

Fluctuation at steady speed? Well if your driving the car it's horribly difficult to keep the load and rpm steady. That's the advantage of an eddy current dyno as it maintains a constant load so very well. Now watching the A/F while in neutral in the driveway is not loading the engine at all but should work well enough to get your idle mix adjusted. As far tuning the idle mix that would be a function of how well you can keep a steady rpm in neutral.

When you say .8 to 1 I think your talking lambda which is better indicator anyways. But for idle and light cruise you want to be a little lean don't you? At least after the engine is warmed up to normal operating temperature. So I think you want a lamba of a little over 1 until you go WOT.

Those Carter AFB copy Edel's should be about as consistent as a carburetor gets.

[travis]

Not lambda, although I do have that option. This is the actual a/f ratio.

I started with watching the afr at idle speed in park. Slight tweaks on the mixture screws can get an average of 14.7, but it’s fluctuating between like 14.2 to 15.2, constantly. Adjusting the idle mix screws can easily move the afr around, but the fluctuation range moves with it as well if that makes sense. Same thing checking idle afr in gear, and the same thing cruising at say 2500. No matter what mixture screw settings or what metering rods I try, that .8 to 1 afr range fluctuation is always there.

I can certainly understand the afr’s moving around when trying to drive at a steady speed, especially as shitty as the roads are around here.

I did notice that setting the idle screws for max vacuum shows the afr around an average of 13.0-13.3. Tweaking for a 14.7 idle afr drops the idle vacuum about .5-1”. Stoich is the goal for idle, isn’t it?

[travis]

Played with it more this evening. Was able to get much less a/f fluctuation (about .3) and less idle rpm fluctuation by running the passenger side IMS about 1/2 turn richer than the drivers side. I’ve noticed this same thing on most every Edelbrock carb I’ve ever ran.

14* initial, and an idle afr of about 13.2 is what it likes for all around starting, idle, and drivability. Any leaner at idle, and it loses a lot of vacuum and gets “ratty”, and much harder to start. Cruise is still fat and WOT is still a bit lean. I’ll work on that more tomorrow.

[Geoff2]

I have found the best place for a/f gauge when setting idle mixture...is in the cupboard.

Holley makes more power than the Edel Carb? Hard to believe. Have you checked to see if the Edel is getting WOT?

[travis]

I have found the best place for a/f gauge when setting idle mixture...is in the cupboard.

Holley makes more power than the Edel Carb? Hard to believe. Have you checked to see if the Edel is getting WOT?

I know, I know lol

That’s probably good advice right there

[PRH]

Imo, Having the motor run “right” trumps any preconceived idea of what I think the A/F gauge should be reading.

I remember talking with a guy at the track who I had done some heads for.
He had bought an A/F gauge and was working on his tune up with it.
He kept leaning the motor out in an effort to arrive at the A/F ratio number he “thought” would be best.
Each step he made in that direction just slowed the car more.

He had a hard time grasping that the motor just wanted what it wanted, and the time slip was telling him he was going the wrong way.

[cardo0]

Not lambda, although I do have that option. This is the actual a/f ratio.

I started with watching the afr at idle speed in park. Slight tweaks on the mixture screws can get an average of 14.7, but it’s fluctuating between like 14.2 to 15.2, constantly. Adjusting the idle mix screws can easily move the afr around, but the fluctuation range moves with it as well if that makes sense. Same thing checking idle afr in gear, and the same thing cruising at say 2500. No matter what mixture screw settings or what metering rods I try, that .8 to 1 afr range fluctuation is always there.

I can certainly understand the afr’s moving around when trying to drive at a steady speed, especially as shitty as the roads are around here.

I did notice that setting the idle screws for max vacuum shows the afr around an average of 13.0-13.3. Tweaking for a 14.7 idle afr drops the idle vacuum about .5-1”. Stoich is the goal for idle, isn’t it?

If you have time to experiment you could try a timing change as you change the IMS. I'm reading conflicting advice so you would need to try both advancing and retarding the static timing to find out for your self. Now I think it's important to have the motor fully warmed up for this as a cold motor will definitely want more fuel.

[Nut124]

Is this an aftermarket A/F system?

Where is the sensor located in the exhaust?

I have been running an A/F system in my car for 20 years with big Webers and do find it useful. However, I do not worrry about idle or part throttle readings. I set those for good driveability w/o being excessively rich. I use the A/F for full throtttle, power tuning only. You can also use it for steady hwy cruising speed tuning.

[PackardV8]

Agree, the A/F ratio is just a number. Give the engine what it wants for timing and mixture to give the best idle in gear, then record whatever the A/F meter gives you for future reference.

Cruise is still fat

You, of course, are aiming to get steady state cruise several numbers leaner than stoich, correct? FWIW, I'm running 2x4-bbl Rochester 4Jets on a Packard V8. The cruise just happened to hit on 14.7; I know I could improve the cruise fuel economy by getting it down toward 16, but it ain't that broke.

[travis]

This is a AEM system, with O2 sensor bungs in the collectors.

I’d be happy at this point to get the cruise close to stoich, as the best I can get with the fattest primary metering rod is 13.0. I’m going to have to juggle some jets and metering rods to see if I can make it better. I did manage to get the WOT to the 12.8-12.9 range.

There is definitely a lot of conflicting info out there about tuning with these things. Everything I have read, including the paperwork that came with this setup, says aim for stoich at idle. I can hit it, but the engine definitely isn’t happy with it.

[Belgian1979]

The fluctuations are due to individual cilinders running leaner/richer than others and since the sampling rate of the sensor is actually quite high, it will read a particular engine at one time and another the next time. This effect is less the higher the rpm.

[F-BIRD'88]

The idle afr on any carbed engine will want to be richer than stoic (14.7:1).. Don't know what you are reading....but...
Set the idle for best idle quality, then very slightly lean the idle screws a hair bit. It will still be richer than 14:7..

The cruise afr can be at AROUND 14.7 ISH..
LET it have what it wants..
Slightly leaner jetting at cruise helps gas mileage
but do not get carried away ..
They don't call it "lean BURN" for nothing.
Any reading is at best a rough average of the 8 cylinders.
They never all run the same. It is the leanest one you watch.
(Sparkplug and ex port appearance)
For WOT give it the jetting it wants to go fast.
The AFR gauge is only used as a YOU ARE TOO FRIGGIN LEAN... warning device. If it wants 11.7:1 afr at WOT thats what it wants.
Even tiny exhaust leaks effect the afr reading.
Leaks and ignition faults show leaner and jumping AFR or both.

[David Redszus]

Anyone who thinks they can achieve a smooth lambda trace even at steady state full throttle is dreaming.

A/F mixtures vary, not only from cylinder to cylinder, but also from cycle to cycle.

The first issue is to be sure the sensor is properly installed and operating at the specified temperature.
Second is to select the sample rate of the data logger. A low sample rate might look smoother but it is not; it is being dampened by the sample rate. Select a sampling rate as high as the system will allow.

At this point the lambda trace plotted against rpm will look like unreadable hash; that is reality. The A/F is actually varying rapidly.
By changing the lambda display from a numerical value or time series trace to a histogram, the distribution of rich and lean spikes will become apparent. You will see a wide range of mixtures with a central tendency. That is the lambda values most often produced by the engine.

Now we must determine what value we are targeting. Forget about a stoich value of 14.7 -1. That is merely a theoretical value and is
different for each specific batch of fuel. Change fuel ratio at full load until the engine runs best. The ignition timing must be changed for torque to be optimized for each jetting setting.

Last but not least, plot lambda against throttle polition, especially during the partial throttle range. This is where stumble occurs when coming off corners.

[PRH]

When I get a carb set up so the idle is strong, and the tip in response is nice and sharp...... the A/F gauge isn’t ever reading mid-14’s at idle.
Mid-high 13’s is more the norm........ with lumpier cams being a bit richer still.

I’m not worried about getting every last .1 mpg out of these things.
I want them to feel like they really run “right” from behind the wheel....... and lean doesn’t “feel” good to me most of the time.

Before all the O2 stuff was available....... they just got tuned to run good.
I still strive for the “runs good” above all else.

One of my old beater DD cars had a 318 2bbl in it. It was very early in the beginning of the smog era. It was a 1975 Fury Wagon.
It had egr and a catalytic converter.
It had extremely poor response when just cruising around, or pulling a hill, trying to pass a car, etc.
You figure, eh...... it’s just a tired 318..... that’s what you’re going to get.
I removed and blocked the egr, removed the cat...... both of which helped, but it still had that wheezing lean feel to it.
I think it was some kind of tiny Carter 2bbl on it, that had a tapered metering rod that was mechanically lifted out of the jets, relative to throttle position.
There was an adjustment so you could raise or lower the rod relative to the throttle position. Just loosen a screw and move the arm on the shaft, retighten screw.... done.

I barely moved it at all in the “richer” direction, and it made a huuuuge difference in the manners and response of the motor.
I don’t remember if it had any impact on the mpg or not....... but whatever it might have been...... it was worth it.
With that little extra bit of fuel....... the motor was actually kinda snappy.