Help with AFR reading

dz01 · Nov 14, 2024

I recently install an ARF to help with carb tuning on my 68.

Background:

car overall runs good.
fires right up but struggles to idle without me feathering for a few mins (I have a manual choke 4100 just rebuilt by All American). Once I get high idle, works fine and idles fine when warm.
car seems flat to be anything over 3k rpms. That’s why I wanted to see what carb is doing with the ARF.

The digital AFR bounces around quite a bit but here’s what I think I have.

Idle- between 13.7-14.4.

WOT- hard to tell but I’d say I’m in the 12’s but will jump to 14’s pretty fast

deceleration (foot off gas in 4th)- 14-14.4

I’m really never higher than mid 14’s.

not that I care about fuel mileage but I am getting 7 mpgs. If my 289 was pushing 400hp I wouldn’t care but for a few mod motor (cam, .30 over, headers) I would expect to get 15 or more. I am playing the timing game where at 10 I was getting hard starts when hot so bumped to 12 with 38 total. No pinging and seems to run fine other than lack of balls over 3k. I’m not as seasoned as most of you are so still learning. Fuel mpgs and seat torque just seems off to me

Blackhawkxx · Nov 14, 2024

dz01 said:
WOT- hard to tell but I’d say I’m in the 12’s but will jump to 14’s pretty fast

If you can get someone to ride with you and video the AFR gauge, it might help you. Have you checked your total timing? Unhook vacuum advance and check the timing at 3K and higher. Timing does play a large role in things.

Noobz347 · Nov 14, 2024

Idle- between 13.7-14.4.

It's idling a little rich.

WOT- hard to tell but I’d say I’m in the 12’s but will jump to 14’s pretty fast

12s at WOT is throttle is good. It [should] lean out [immediately] when you take your foot off the gas.

deceleration (foot off gas in 4th)- 14-14.4

This is fine. Might be a little fat but it's within the margin of error. If you lean it out at idle this will change too.

I’m really never higher than mid 14’s.

Alluded to this already but somebody that knows carb jets better than me might be better at advising how to best adjust.

Noobz347 · Nov 14, 2024

Blackhawkxx said:
If you can get someone to ride with you and video the AFR gauge, it might help you. Have you checked your total timing? Unhook vacuum advance and check the timing at 3K and higher. Timing does play a large role in things.

...and along these lines:

Does your AFR meter have the ability to log data?

dz01 · Nov 15, 2024

Noobz347 said:
...and along these lines:

Does your AFR meter have the ability to log data?

Not the one I have

LILCBRA · Nov 15, 2024

dz01 said:
Idle- between 13.7-14.4.

WOT- hard to tell but I’d say I’m in the 12’s but will jump to 14’s pretty fast

2 questions about your idle: what is your idle vacuum and have you tuned the idle circuits to obtain the most vacuum that you're able to get?

1 suggestion about WOT - it seems like your discharge nozzles may either be too big or the pump timing needs adjusting. Also, I don't know much about the 4100, but you could try bumping the jets up a couple sizes to maintain a lower AF reading while the throttle is still wide open. Otherwise your ratios don't sound too horrible. What are they at part throttle cruising? The rough guidelines I went by when I was messing with my Holley were:

Cruise - I shot for something close to 13.5-14.0
WOT - somewhere around 12.5
Idle was wherever it ended up, but it was usually a little lean - somewhere between 14.7-15.x iirc. But as was pointed out to me, idle is set to the highest vacuum you can get out of the engine.

You'll have to keep in mind that those numbers are going to vary no matter what you do, especially when the weather differs wildly from day to day. I never did completely finish tuning my Holley before I bit the bullet and got myself a throttle body injection setup, but you can reference my thread on tuning the Holley with the AF gauge, maybe it will help you some.

Thread '14point7 Spartan 2 install, Holley 4150 tuning, & exhaustcapades'

Apr 22, 2020

As I said in the BS thread, here is my thread on installing the A/F setup and gauge to try and tune my carburetor. As many of you know, I swapped manifolds a while back and replaced the Offenhauser 360 with a cheap Edelbrock Performer RPM Airgap knockoff a couple years ago and during that process I took the carburetor back to off the shelf specs. I didn't do much to it after that, hell I've hardly driven the car! I've been too preoccupied with other things in life, but now things are starting to fall into place (other than this COVID-19 mess we're going through right now....) and I'd...

limp · Nov 16, 2024

Are you timing it with the vacuum advance hose disconnected

dz01 · Nov 17, 2024

LILCBRA said:
2 questions about your idle: what is your idle vacuum and have you tuned the idle circuits to obtain the most vacuum that you're able to get?

My vac at idle is around 12. I have an unknown cam so I’m not going to pull 18-20.

LILCBRA said:
1 suggestion about WOT - it seems like your discharge nozzles may either be too big or the pump timing needs adjusting. Also, I don't know much about the 4100, but you could try bumping the jets up a couple sizes to maintain a lower AF reading while the throttle is still wide open. Otherwise your ratios don't sound too horrible. What are they at part throttle cruising? The rough guidelines I went by when I was messing with my Holley were:

I think cruising in at around 13

LILCBRA said:
Cruise - I shot for something close to 13.5-14.0
WOT - somewhere around 12.5
Idle was wherever it ended up, but it was usually a little lean - somewhere between 14.7-15.x iirc. But as was pointed out to me, idle is set to the highest vacuum you can get out of the engine.

You'll have to keep in mind that those numbers are going to vary no matter what you do, especially when the weather differs wildly from day to day. I never did completely finish tuning my Holley before I bit the bullet and got myself a throttle body injection setup, but you can reference my thread on tuning the Holley with the AF gauge, maybe it will help you some.

Thread '14point7 Spartan 2 install, Holley 4150 tuning, & exhaustcapades'

Apr 22, 2020

As I said in the BS thread, here is my thread on installing the A/F setup and gauge to try and tune my carburetor. As many of you know, I swapped manifolds a while back and replaced the Offenhauser 360 with a cheap Edelbrock Performer RPM Airgap knockoff a couple years ago and during that process I took the carburetor back to off the shelf specs. I didn't do much to it after that, hell I've hardly driven the car! I've been too preoccupied with other things in life, but now things are starting to fall into place (other than this COVID-19 mess we're going through right now....) and I'd...

LILCBRA

Replies: 143

Forum: 1974 - 1978 Mustang II Talk & Tech

GOvert · Jan 8, 2025

Are your back barrels functioning?

Help with AFR reading

dz01

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Blackhawkxx

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Noobz347

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Noobz347

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dz01

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LILCBRA

I wish I didn't have all of these balls in the air

Thread '14point7 Spartan 2 install, Holley 4150 tuning, & exhaustcapades'

limp

wrap a little cheese around it and its a done

dz01

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Thread '14point7 Spartan 2 install, Holley 4150 tuning, & exhaustcapades'

GOvert

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Help with AFR reading

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Advanced Member

Stangnet Facilities Maint Tech... Er... Janitor

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I wish I didn't have all of these balls in the air

wrap a little cheese around it and its a done

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