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Re: [Fot] Curious Air Fuel Ratio numbers

To: Jim Gray <>
Subject: Re: [Fot] Curious Air Fuel Ratio numbers
From: Jack Mc <>
Date: Sun, 23 Jun 2013 01:11:43 -0400
Cc: Triumph Friends of <>
References: <>
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

Jack Mc

Sent from my iPad

On Jun 22, 2013, at 4:01 PM, "Jim Gray" <> wrote:

> Greetings FoT
> I'm tuning a fresh motor and am getting some strange numbers on my AFR
> ( 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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