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Old 05-22-2012, 02:33 PM
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tuning my air/fuel ratio? carb

I want to fine tune my air fuel ratio on my q-jet. I have a edelbrock (narrow band) gauge that I read is pretty useless. I will be installing new plugs since it is a new build and just getting past the break in I dont know how accurate the markings will be on those. Where to start? thanks

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Old 05-22-2012, 02:58 PM
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A narrow band type AFR gauge even a simple three light like yours is very very usfull if you know what it is telling you.

You can also read the O2 sensor with a good volt meter.
You want a heated type sensor so it reads at idle.

These gauges are very accuate at showing you when you are at, near or just crossed over the stoichiometric ratio point (14.7:1)

When tuning the idle, off idle, main jet part throttle cruse tuning , accelerator pump action this is invaluable. WOT best power jetting is found by drag strip (MPH) or dyno testing and this meter will clearly show-warn you if you are dangerously lean even thou it does not show you the exact AFR in the rich zone @WOT. Generally you will want to see a WOT voltage of .800 or higher. .750volt @WOT or less means you are getting lean. look at your plugs. Fix it.

use a vacuum gauge and the AFR gauge to set up the idle.
sometimes a bit richer than 14.7:1 is best at idle. best idle manifold vacuum is where you want to be.

Again being able to see the 14.7:1 AFR crossover point as you tune is invaluable.

Its all in how well you can interpret the gauge info.

Exhaust leaks will throw off the guage. Missifre shows up as a leaner flittering gauge voltage @WOT.
Ya a wide band gauge is sweet, but you can do 99% of dialing in your carb with a simple Narrow band gauge, a vacuum gauge, some track-drive testing.
and the Q jet tuning parts to change things.
a single wire O2 sensor needs to be mounted real close to the exhaust port or it runs too cold at idle to be accurate.
( if it cools off on you, you have to rev the motor to get it hot again, to read at idle.) 600F minimum temp
A 3 wire or 4 wire heated type O2 sensor is a lot better for tuning.

And once you are all done tuning, this gauge wll clearly show you if the tune has CHANGED at all. Once you know what normal gauge readings are at idle , cruise and WOT
Its a lot more usefull than you think.

The AUTO Meter 20 led gauge is a bit better (more resolution, more like reading the actual voltage output) but the edelbrock 7 light gauge will get it done.

You can check compare the voltage output that lights each LED with a good volt meter.

If you have a good volt meter that has a PC interface (serial or USB port)
you can log the O2 sensor voltage output over time on a PC with free downloadable software.






Did you ever consider reading the directions?

Last edited by F-BIRD'88; 05-22-2012 at 03:26 PM.
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Old 05-22-2012, 04:09 PM
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thanks for the help. I am sure I read the directions about 10 years ago. I just got this thing out again and while researching and asking on a couple different forums all I got was they are only good for WOT. This vehicle does not really see WOT but I am confident that WOT is rich. This is a daily driver and 98% of the time I am in the cruise circuit which I am trying to dial in.
The sensor that comes with it is a 3 wire , I am checking to see if it is a heated one or not. I had a few links in years past on doing it with a voltmeter but will have to find those again.

I am going to hook it up again this evening. I had it on there last year with the same set up only the engine was not rebuilt yet, same cam, set up,etc. While getting on the highway I was at the 14 mark LED, accelerating. After coming to speed though the lights would slowly go all out. I was a little hesitant on richining up the cruise circuit though after reading these gauges only do well for WOT?

Last edited by Catfish11; 05-22-2012 at 04:23 PM.
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Old 05-22-2012, 05:10 PM
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Quote:
Originally Posted by Catfish11
thanks for the help. I am sure I read the directions about 10 years ago. I just got this thing out again and while researching and asking on a couple different forums all I got was they are only good for WOT. This vehicle does not really see WOT but I am confident that WOT is rich. This is a daily driver and 98% of the time I am in the cruise circuit which I am trying to dial in.
The sensor that comes with it is a 3 wire , I am checking to see if it is a heated one or not. I had a few links in years past on doing it with a voltmeter but will have to find those again.

I am going to hook it up again this evening. I had it on there last year with the same set up only the engine was not rebuilt yet, same cam, set up,etc. While getting on the highway I was at the 14 mark LED, accelerating. After coming to speed though the lights would slowly go all out. I was a little hesitant on richining up the cruise circuit though after reading these gauges only do well for WOT?
no you got it bacwards

@WOT the gauge is essentually a WARNING device.
It tells you its too lean if the gauge goes leaner than the red bulb.
You pretty much want it to stay pegged red when at WOT.

At cruise it is very good and telling you stuff.
If the gauge LEDs goes out at cruise it is way lean or way rich. or the heater is fubared. too cold
usually its way too lean when off the scale.

tune for more or less 14.7:1 at cruise and 13.2:1 or richer at any time the car is accelerating.

Q jets are very specific and need exact tuning. When they are right, they are very right and when they are off tune they are very off the mark.
Again any exhaust leaks for or aft of the O2 sensor will throw it off.
Any all vacuum leaks must be fixed too.
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Old 05-23-2012, 01:36 PM
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I took the truck out for a spin earlier and heres what I came up with.

Turn the truck on , choke is engaged, and the LED's go to the last red on the right and goes out. LEAN
I take the choke off and idle and the LED's light all up coming from the right hand side and stop at 12.5
Accelerating on to the highway , slow and steady acceleration LEDs are lit up from the right to the 13 bulb. RICH
WOT, and the bulbs are lit up from the right to the 12.5 mark. RICH
Letting off the gas completely after WOT and I get 12.5. Rich




I am thinking I should start with my jetting, to get WOT around 14.7 then start on the cruise circuit?

Last edited by Catfish11; 05-23-2012 at 01:47 PM.
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Old 05-23-2012, 01:44 PM
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Quote:
Originally Posted by Catfish11
I took the truck out for a spin earlier and heres what I came up with.

Turn the truck on , choke is engaged, and the LED's go to the last red on the right and goes out. LEAN
I take the choke off and idle and the LED's light all up coming from the right hand side and stop at 12.5
Accelerating on to the highway , slow and steady acceleration LEDs are lit up from the right to the 13 bulb. RICH
WOT, and the bulbs are lit up from the right to the 12.5 mark. RICH
Letting off the gas completely after WOT and I get 12.5. Rich


I am thinking I should start with my jetting, to get WOT around 14.7 then start on the cruise circuit?
WOT must be 13:1 or richer ,,, NOT 14.7 14.7 is too lean @WOT.

14.7@WOt will burn up the motor.

You are lost on this. WOT must be 12.5:1 to 13.2:1 no leaner.
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Old 05-23-2012, 01:46 PM
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In a earlier post I said the red LED was the rich LED. It is not The RED LED is the leanest LED.

The two Yellow LED s are your rich LED's should be lit @WOT.
one or both of the yellow LED should be lit @WOT.

If its in the Green @WOT you are too lean @WOT.
read the directions.
read the tuning manual for the edelbrock carb.

The green zone is the part throttle cruise AFR ZONE.
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Old 05-23-2012, 01:50 PM
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Ok, good so my WOT and acceleration are ok. I need to work on enriching up the cruise circuit, right?
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Old 05-23-2012, 02:14 PM
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Quote:
Originally Posted by Catfish11
Ok, good so my WOT and acceleration are ok. I need to work on enriching up the cruise circuit, right?
No , steady cruise driving would be any of the green LED's
A slightly lean AFR at cruise will save a bit of gas.

The idle is set with a manifold vacuum gauge
14.7:1 or a bit richer (13.5:1)

open the throttle just a bit ( high idle speed to 2000rpm hold steady should be in the green zone. This is the idle circuit, not the main jet.

The main jet /rod afr is found at 30-45MPH at steady speed cruise.
2000 to 3000 rpm at a fair and steady road speed.

low speed driveing is the idle and off idle circuit

The engine must be fully warmed up before setting the idle mixture
or evaluating jetting. can take up to 15 minutes of driving to warm it up.
Go drive the car before messsing with it.

any time you roll onto the throttle it should shift to richer.
If it first goes leaner, then goes richer or bogs, the accelerator pump needs adjustment or the power step spring is wrong.
You should be able to see the power step spring work with the vacuum gauge and AFR meter and throttle input from steady cruise into power as you apply throttle.

You can read the O2 sensor with a volt meter. Should be the purple wire on the
O2 sensor connector.

The O 2 sensor must heat up a bit to read at idle.
Whe using the choke to warm the engine from cold do not allow the engine to idle slow. 1200-1500+rpm cold idle speed untill it get some heat going on.
Ignore the gauge reading until its warmed up.
cold engine random cylinder missfire shows lean on the gauge or off scale.

You seen dyslexic. Got things bacwards a bit.

idle afr 12.5 to 14.7:1 12.5 is ok but a bit leaner is a bit cleaner ( wam engine)
lower CO output.

of idle low speed 13:1 to 15:1 is ok 14.7:1 best emissions 15.5:1 best fuel mieage at cruise.

high speed steady cruise 14.7:1 +/- a bit a bit lean saves fuel
60 to 70MPH steady

WOT 12.5:1 to 13.5:1 RICH

WOT is found by power testing remember the guage only tells you if you are rich. Not how rich. There is a difference.
Its just a rough guide and warning device at WOT.
The volt meter will show a bit better.

remember the gauge is VERY sensitive right at the 14.7:1 crossover point.

Just like the AFR/Voltage chart shows .

reset the idle when warmed up ( idle mix screws) 14.7:1 or slightly richer ( best idle vacuum)

Go drive the car.

Last edited by F-BIRD'88; 05-23-2012 at 02:24 PM.
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Old 05-23-2012, 02:24 PM
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NEVER EVER EVER use an O2 sensor for tuning accel enrichment nor anything at idle. When it comes to that it will actually send you in the wrong direction. It is good for WOT and cruise tuning though.

For idle the easiest thing to use is your vacuum gauge, for accel enrichment you can use your acceleration rate if you can datalog it. Otherwise its the old fashioned methods.
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Old 05-23-2012, 02:35 PM
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NEVER EVER EVER use an O2 sensor for tuning accel enrichment nor anything at idle.

this is not true. Its a matter of being able to interpret the info.

These gauge do not actually show you the AFR. They detect and show you unburned O2 in the exhaust.. That means something.

Warm the enigne up a set the idle screws so it idles at 14.7:1.
Then using the manifold vacuum gauge find best idle manifold vacuum and rpm
and idle quality. It will be just slightly richer than 14.7:1 (12.8 to 13.8 usually)
Now roller the idle screws just a bit leaner towards 14.7:1 till the idle rpm just drops a hair. thats the sweet spot.

Now Open the throttle to increase the idle speed and watch the AFR gauge.
It should stay very close to 14.7:1 +/- ( basicly the green zone)
The red bulb may flicker. thats ok 15:1 ish AFR.
Test the throttle response and watch the gauge.

The engine must be fully warmed up with no vaccuum leaks or ignition faults.
Misfire makes the gauge leaner or flUtters or both.

Last edited by F-BIRD'88; 05-23-2012 at 02:46 PM.
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Old 05-23-2012, 02:43 PM
How fast is fast enough?
 
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Quote:
Originally Posted by F-BIRD'88
NEVER EVER EVER use an O2 sensor for tuning accel enrichment nor anything at idle.

this is not true. Its a matter of being able to interpret the info.
The info will be all over the place at idle, not even close to correct, and the info under accel will be delayed, but not delayed consistently. Unless you can log it all to compare after the fact its useless for accel. Even if you can log it its useless for idle.
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Old 05-23-2012, 02:47 PM
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Quote:
Originally Posted by ap72
The info will be all over the place at idle, not even close to correct, and the info under accel will be delayed, but not delayed consistently. Unless you can log it all to compare after the fact its useless for accel. Even if you can log it its useless for idle.
Not on my motor. But mine is dialed in and fireing on all 8.
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Old 05-23-2012, 02:49 PM
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Exhaust leaks will throw the guage reading off.

Compare the volt meter reading to what each LED switch on point is.

Now play with the idle mix screws and watch the volt meter and LEDs
You will see that the engine will actually idle quite will thru a wide AFR range on the gauge. Best idle manifold vacuum and idle smoothness is the priority.
You will see that that best idle point is just north of 14.7:1.

Remember @WOT is is just a basic Warning device.
to help keep you from going too lean @WOT.
You will learn what normal reading s are by using it.

best WOT jetting is found by Drag testing for MPH or dyno testing.

Last edited by F-BIRD'88; 05-23-2012 at 03:08 PM.
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Old 05-23-2012, 03:04 PM
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Quote:
Originally Posted by F-BIRD'88
Not on my motor. But mine is dialed in and fireing on all 8.
You must have one of those magic engines with complete combustion in the chamber and perfectly matched intake and exhaust distribution... you should show the car manufacturers how to do that as they still don't use the o2 readings at idle nor under accel even with all their fancy engine management.
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