I just installed an Auto Meter Air Fuel ratio guage (using Auto Meter Oxygen Sensor) on my carbed 460 in my 79 F250 and I'm unsure of what it should be reading, when. Upon initial hookup, I found that at idle I was almost max'd out on richness on the guage. I switched to leaner metering rods in the carb and I'm still rich, but closer to stoich (2 led bars away)... so I here are my questions:

First question: I presume that the meter should read rich at idle since the motor needs to run richer at idle to prevent stalling. Is this correct?

Second question: At what point should the engine be operating in a stoich state?

Under hard acceleration, the guage goes to extreme lean, at some points its so lean, the guage shows no reading. At cruise the led's jump wildly between lean and rich -- I never see the guage in a constant stoich state -- not sure if this is correct or not.

Autometer's instructions are not much help on a carb'd motor, all of the content refers to ECU activities and their influence on the meter.
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stanz
74 Bronco Sport, 302 Stock
79 F250 Trailer Special, 460 4x4 Extend Cab

 

The metering rods should not effect the idle much. You will need to adjust the mixture screws on the carb for that.

 

Hi,
Interesting subject. I have often thought of trying one of these out.

As to your first question. No, I don't believe so. As LxMan1 states above if your idle mixture screws are set correctly your A/F ratio should be at that golden 14.7:1 and the meter should reflect that.

Question#2. I would think that you will be nearest that state anytime the engine is operating at a constant RPM will as little load as possible.

From what I have read installing one of these meters on a non-ECU carb'ed engine only has an advantage of helping you "tune" your carb to your engine/truck combo. As you have stated, the readings will be all over the place during normal driving as you don't have a computer "brain" to constantly adjust A/F ratio like an injected engine does. The meter may help you set you idle mixture as explained in question #1 above. The only other instance I know of that you can use it is for setting your "cruise" A/F mixture by changing the main jets. This is what I have read on doing just that.

Drive along a flat, straight, long stretch of road at around 45mph and keep the speed as constant as possible. Use cruise control if ya have it. Record what the meter displays. Hopefully the reading will be stable enough to read. The object here is to change the jets(one size up or down at a time) and repeat the test till you get the meter reading you desire. You don't want to go too lean as engine knock and damage could result.

Bear in mind that this is only what I have read on the net and also that I have never tried this.
Good luck and post your results,
Lee

 

Lee and LXman, thanks for your responses.

One thing I failed to mention in my first post was that with both sets of metering rods, I set the A/F mixture screws to the point of highest vacuum (19.5 lbs).

I found that I could not seem to lean out the mixture to stoich and have it run good. In fact, I couldn't keep it running for more than 10 secs. on anything that didn't register as rich on the guage. Since I the carb was on the vehicle when I purchased it, I suspect the jets are way rich -- I know the previous owner claimed to have tuned to the carb (presently running a Carter 625).

I'm likely going to opt up to a new Edelbrock 750 this week anyway -- Carter could use a rebuild and would like to up the cfm's a bit, and it will give me a solid baseline for tuning.

So if I understand correctly, the meter should read Stoich (approx. 14.7) at idle, and the same at a steady cruise? So when I set up the new carb, this is what I should be aiming for.

Another question in my head... what effect does timing have on stoichometry? I presume that it also could be a contributing factor. I'm not sure I've found optimal timing yet either. I'm perplexed over what to work on first, the timing or the mixture.

Thanks in advance.
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stanz
74 Bronco Sport, 302 Stock
79 F250 Trailer Special, 460 4x4 Extend Cab

 

Check this out.

http://hp.autometer.com/techtips/faq.../air_fuel.html

It talks about the meter's behaviour on fuel injected motors but it is still helpful. Since the oxygen sensor's output is not linear, it is normal for the readout to bounce back and forth if the A/F ratio is close to ideal. I would adjust if the meter read constant low or high. I think that the bouncing you have means your ratio is good.

 

I checked out the Autometer link and it says nothing about idle. I spoke to both Summitt Racing and Auto Meter tech and both said that carb'd motors, especially big blocks, should run rich at idle. They both agreed I'd never get it to run at stoich at idle.

So from what I've learned, I should aim for a steady, slightly rich reading at idle by setting mixture the old fashioned way using a vacuum guage. The reading on the guage is now my baseline for any deviations.

So in sum, the guage is cool to look at, but doesn't help too much with tuning other than telling you if you're way off, which I think most of us are capable of telling without a guage.
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stanz
74 Bronco Sport, 302 Stock
79 F250 Trailer Special, 460 4x4 Extend Cab

 

Honestly, I have never seen one of those units work as well as, say, an FJO or LM-1. If you're going to tune using a wideband, then I would recommend getting either one of these instead. They are a little more $$ but they are much more accurate for tuning. They also have the ability to data log so you can see what you're fuel curve looks like through the entine RPM range, and they show a number (down to the thenth) instead of a lighted bar.

 

I am using an AF guage on my 400. It took a little while of watching it an fiddlng with my mixture to really understand what it was showing me. Especially when you read the carb tuning part of say, the edlebrock carb manual that says basically to make it as lean as you can without driveability issues.

Here is where I am at now, and I think I have it as close as a carbed motor will get.

At idle, I did try using the rods and jets to get the gauge too read a nice stoich, but it left me lean while actually driving and idled rough. I did as you have, and used the vacuum gauge.

At cruise, it is a balance between just driving at a steady speed and accelerating before the power mode kicks in. I could get the steady speed mixture nice and stoich, but as soon as I gave it a bit of gas, it would go lean, even to point of not even lighting a bar on the gauge any more. (this is where an EFi shines). I found I had to adjust the rod/jet to be just on the rich side of stoich ( I have one green and one yellow bar glowing) while going steady, when I start to give it gas, it will flutter across the stoich range of the gauge, but not hover down in the lean part. A computer can control the mixture fast enough to make that gauge hold steady, the carb is an average. Tracking on edelbrocks mixture chart for my carb, I am running about 2 stages rich on cruise according to them, but my engine likes it. If I went leaner, I had pinging issues under light acceleration.

When the rods lift, and the carb goes into power mode, the gauge pops leans for a second, then goes back to slightly rich, lighting the last yellow bar, and the first 2 green ones. The adjustment behaved the same as cruise mode, and I had no pinging issues if I kept it a hair rich according to the gauge.

When the secondaries open, the gauge again pops lean, then jumps back into the same area as when in power mode. I may yet experiment with enlarging my jets and see if I can notice a difference.

In summary, when you see that gauge bouncing from rich to lean, you are hovering right in the stoich range, the carb isnt accurate enough to get closer than that with a narrowband gauge. It is normal to have a couple bars lit at the same time, you just have to look at the average of them. I am running with 1 yellow and 1 or 2 green bars lit, which tells me I am on the rich end of stoich, but real close to being right on the money. The power setting and secondaries are easier to adjust since you dont really have to balance steady speed and slight acceleration mixtures like you have to do in cruise mode. You want those to be slightly rich for better power. The cruise mode is a balance between steady speed, and light acceleration performance.

"First question: I presume that the meter should read rich at idle since the motor needs to run richer at idle to prevent stalling. Is this correct?"
I agree. I have heard that at idle there isnt enough exhaust flowing to accurately get a reading.

"Second question: At what point should the engine be operating in a stoich state?"
Tune cruise mode to be stoich, tune power mode and secondaries for performance.

"Under hard acceleration, the guage goes to extreme lean, at some points its so lean, the guage shows no reading."
Yep, its lean.You need to enrich your power mode.

"At cruise the led's jump wildly between lean and rich -- I never see the guage in a constant stoich state -- not sure if this is correct or not."
With the narrowband gauge this is normal, when it is lighting up like a christmas tree, you are real close to stoich.

 

I have a question to throw on to the end of this... Being that my truck has true duals, if I wanted to get one of these gauges, would I have to have 2 O2 sensors or just have one in one pipe and assume that the other pipe is the same?

 

Either way, I put one on the collector of my header, but you can put one on each side and either use 2 gauges, or hook up a switch to toggle back and forth.

 

Correct. Even Autometer's instructions suggest using 2 O2 sensors and a toggle switch if you want to measure both sides.
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stanz
74 Bronco Sport, 302 Stock
79 F250 Trailer Special, 460 4x4 Extend Cab

 

Stanz, how is your tuning working out?

 

Bee Boy-
Thanks for asking. I got the current carb tuned as well as it can be. My major issue is that it needs an accelerator pump and some jetting. I wanted to up the cfms so I ordered a remanufactured Edelbrock 750 from Summit. I would have had it on already, however, they've sent me the wrong phenolic spacer 3 times now. The next one is set to arrive this morning via express via overnight UPS. If all goes as planned i'll have the new carb on today and the meter will advise on any metering/jetting I need to do.
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stanz
74 Bronco Sport, 302 Stock
79 F250 Trailer Special, 460 4x4 Extend Cab

 

Cool, I'm curious as to how it works out for you.

 

Got the carb on, but now with the 1/2 in. spacer, the throttle cable is 1 inch too short and I need to do some bending fo the kickdown rod. Where can I find a longer than stock throttle cable?