Idle Mixture and AFR

[Vetman]

On my 75 I have the Innovate AFR measurement dual gauge system and a Holley 600cfm carburettor. My usual 40 minute joy ride was showing an average AFR of 13.0. Richer than Stoic but I had my foot down for part of the way. I closed the two idle mixture screws by 1/8 turn and on the last ride recorded an average AFR of 14.0. Traffic and cruising at very little throttle will use the idle rather than the main jet circuit. The plugs looked a better colour.
Not sure if there is a direct relationship between average AFR and fuel usage but if so then about 7% fuel saving for a small adjustment.

 

[Roscobbc]

Thats an interesting observation - the carb on my engine being a race spec' double pumper is really quite difficut to tune for an ideal idle - different between lean and rich is barely 1/8" turn on each of the 4 chokes. The transition period where it comes-off idle jets and goes on to mains is around 1200 rpm in 4 gear.....very dependant on ambient/engine temperatures - too hot and it runs a little lumpy......ideal temp is 180/185 F. Have never done a AFR reading - all set-up by 'ear' and plug observation.

 

[Vetman]

Good plug colour is a good indicator.

 

[teamzr1]

Need to account if and how much Ethanol percentage being used.
The old days of Stoch being 14.7:1 went away when E10 came out as it leans the piss out of cylinder charge

GM sets all non WOT conditions as 14.2:1 and best case 12.6:1 for WOT

Where that 9.0 come from ?

 

[Vetman]

Need to account if and how much Ethanol percentage being used.
The old days of Stoch being 14.7:1 went away when E10 came out as it leans the piss out of cylinder charge

GM sets all non WOT conditions as 14.2:1 and best case 12.6:1 for WOT

Where that 9.0 come from ?

View attachment 19041

I try to use petrol with no ethanol or very little but accept your point. The range of AFR is because a carburettor is a blunt instrument compared to a mapped fuel injection system. However, the map value is a target and the actual instantaneous AFR value is what it is. The upper and lower values shown occur for very short periods as you come on or off the throttle. The O2 sensor is interrogated several times a second so these values are short lived.

 

[phild]

I must confess I'm a little naive in matters of ethanol effects on engine running, as you all know my 'Vette is not running yet. However I have been using this additive, https://www.millersoils-shop.co.uk/eps-ethanol-protection on my classic bikes with no ill effects although its early days. Mind you they're not exactly high performance bikes, Kawasaki KLR250 '87 vintage and Honda CX500 '83 vintage.

Anyone any experience with additives?

 

[teamzr1]

I try to use petrol with no ethanol or very little but accept your point. The range of AFR is because a carburettor is a blunt instrument compared to a mapped fuel injection system. However, the map value is a target and the actual instantaneous AFR value is what it is. The upper and lower values shown occur for very short periods as you come on or off the throttle. The O2 sensor is interrogated several times a second so these values are short lived.

I understand what you're stating
Point is due to Ethanol that jets of the carb may need to change the flow size to makeup for how much leaner Ethanol makes of the AFR
You're lucky if you can find straight gas and not costing much over Ethanol blended is
Here in the USA finding straight gas is harder to find who sells it and expect to pay $1 plus per gallon over Ethanol based

Also, just a note, IF the vehicle has a true dual exhaust system, and only 1 wideband O2 is used, that means it is only monitoring
one head, and will find there is an imbalance of left to right heads, so what is reported on that head may not be the same AFR on the other head.

Maybe worth having another bung welded to the other side and once in a while move the WB to other side to compare the AFR left/right
and then tune maybe the middle of what both sides AFR reported

Here is one example of monitoring the 2 front O2 sensors and the imbalance seen between left and right heads exhaust makeup
I superimposed the O2s data (overlaid)
See via captured data using an OBD scanner that Bank 1 (B1) at times runs - 6.3 % richer while Bank 2 (B2) runs - 2.3% richer

 

[teamzr1]

I must confess I'm a little naive in matters of ethanol effects on engine running, as you all know my 'Vette is not running yet. However I have been using this additive, https://www.millersoils-shop.co.uk/eps-ethanol-protection on my classic bikes with no ill effects although its early days. Mind you they're not exactly high performance bikes, Kawasaki KLR250 '87 vintage and Honda CX500 '83 vintage.

Anyone any experience with additives?

Looking at that product, its claim is to protect the internals of fuel system that would degrade over time as the parts were not
designed for the effects of what Ethanol does
It though is not making up for the fact Ethanol causes a leaner burn and hotter cylinder charge, so the end result is leaner AFR
You're lucky in the UK is if they are only forcing use of E5, where here in the USA was E10 but now E15,
Only reason is fricking crooked people holding political office that get payoffs by lobbies that profit from selling anything related to adding
ethanol to the gas.

The higher the Ethanol percentage is, the leaner the AFR will be,
inducing more engine knock, reduced timing, less performance and less fuel mileage

In short, it is horsecrap and does not belong in Corvettes

 

[Vetman]

I have a sensor in each of the duel pipes. Very similar AFRs with any higher value swapping between sides. A sustained variation might indicate a problem on one bank of the engine or the need to recalibrate or replace a sensor. Carburation problems should not effect one bank more than the other as each side of the carburettor feeds two cylinders on each bank.

 

[Vetman]

Make that dual. Can I switch off predictive text?

 

[Roscobbc]

Almost mandatory to have dual sensors I'd have thought if running a carb. How else would you 'balance' left and right side of engine?

 

[Vetman]

Almost mandatory to have dual sensors I'd have thought if running a carb. How else would you 'balance' left and right side of engine?

My intake manifold is a two-plane design where one half of the plenum feeds two cylinders on the left bank and two cylinders on the right bank. So any adjustment to one side of the carb will effect both banks/sensors. Would you ever set out to tune each side of the carburettor differently?

 

[teamzr1]

I have a sensor in each of the duel pipes. Very similar AFRs with any higher value swapping between sides. A sustained variation might indicate a problem on one bank of the engine or the need to recalibrate or replace a sensor. Carburation problems should not effect one bank more than the other as each side of the carburettor feeds two cylinders on each bank.

Carb would not cause an imbalance, but is used as to jetting to adjust for actions of Ethanol
But others things can

Here is an example of a 2010 C6 Z06
Owner put some stupid so-called performance mods on it and car ran like crap, so he came to me to debug and then tune PCM up
In analyzing the OBD recorded data, I found this crap

Notice at times B1 was 25% too lean while B2 was -1.6% rich, what a cluster frick that was
This would not happen when people owning a sports car are not monitoring how engine or tranny is functioning

The cause of this and almost screwed his 427 CI engine was, he got sucked into a high dollar mod that is of course to give 200 HP gains
by replacing the stock intake manifold
This type4 was really designed for one purpose, only and that is when using N20 (NOS) if there was a F up that instead of the engine blowing to hell
this intake design is 2 sections and when the big bang happens with NOS that instead the intake comes apart

Well crappy design and what happened due to heat was the intake on the left side had warped at the left rear corner, so there was unmetered air being sucked into that side :-(

25% lean in hot summer weather is asking for a ass banging for sure



Not the carb in your case, but still ill effects from

1. leaking exhaust manifold on one side, pulling in air to cylinder(s)
2. shape of intake manifold causing imbalance
3. If having headers, length of each primary, if lousy design not all same exhaust flow length
4. back pressure, one side flows better than other side
5. cylinders not getting clearing out exhaust fully and not cycle dirty charge in cylinder
6. one side exhaust pulses to help extract exhaust by some issue with exhaust downstream
7. if using WB sensors, one not correct or getting air leak close to it.

If fuel injected and engine controller, other things can fact into why one side's AFR is different

 

[Roscobbc]

My intake manifold is a two-plane design where one half of the plenum feeds two cylinders on the left bank and two cylinders on the right bank. So any adjustment to one side of the carb will effect both banks/sensors. Would you ever set out to tune each side of the carburettor differently?

Yes, possibly on an engine than is known to have unequal cylinder distribution where perhaps one cylinder is known to run weaker than others.......just a case of tuning mixture to weakest cylinder. Some specific 60's/70's performance engines with a 4bbl from oem actually could have different sized main jets each 'side'. In your example above surely setting up from AFR on one bank would still need same process for the opposite bank, read of the other exhaust for a 'balanced' AFR?

 

[Vetman]

If I had a variation in AFR over the two banks I would look for the cause such as exhaust leak, spark plug, lack of compression, valve issue, etc., or dodgy O2 sensor.