Subject: [Fot] Curious Air Fuel Ratio numbers

From: "Jim Gray" <[email-masked]>
Date: Sat, 22 Jun 2013 14:01:32 -0600

Greetings FoT

I'm tuning a fresh motor and am getting some strange numbers on my AFR gauge
( autometer wide band ) 

with Mikuni HSR45 carbs. 

I have a very different engine than last year so I'm starting from scratch
on my jetting this year. 

I just put in a fresh 02 sensor and am seeing AFR of 9.4 at 4k RPM cruise
and WOT from 3K to 6K.

However, when I lift from 6K to zero throttle the air fuel goes to 10:1,
11:1 and 12:1.

The plugs have some color but nothing indicating a mixture that rich. I
haven't done any proper full throttle 

shut downs on the plugs yet, just trying to get closer with AFR's & EGT's

Exhaust gasses haven't gone over 1200F yet. Since the car is registered for
the street I'm just trying to get 

a tune that's close before I get it the dyno for the " real tune "

I haven't ruled out a faulty gauge/afr reading either. I've had new parts
let me down before.

Just wondering if anyone has seen a mixture go lean after throttle lift.
Seems kinda bass ackwards.

Thanks as always,

jim g
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From: Jack Mc <[email-masked]>
Date: Sun, 23 Jun 2013 01:11:43 -0400

Yes, I've seen the mixture "go lean" after throttle lift - it's
counter-intuitive, but it happens when, even though the amount of unburned
fuel goes up, the amount of oxygen not reduced by combustion goes up even more
- so the mixture ratio indicates lean.

I (along with a more experienced business partner) opened a Dyno shop almost a
year ago - one of the first things I learned was not to be misled by this
phenomenon.

Cheers,
Jack Mc

Sent from my iPad

On Jun 22, 2013, at 4:01 PM, "Jim Gray" <[email-masked]> wrote:

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From: "MadMarx" <[email-masked]>
Date: Sun, 23 Jun 2013 19:38:12 +0200

I don't know if this is a help but many of my videos show AF values.

Chris