Trying to figure out AFR reading

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gtxdude

gtxdude

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So on my 3rd sensor going into bung it seems to be working. Have idle about 13.5, going down road it's between 12.2-12.8 according to chart that's rich. Pulled #2 plug and it's looking pretty white like it's running lean. Who do I trust, fancy new AFR gauge or reading plugs?

IMG_20240518_154649964.jpg
 
They should match. If they don’t, believe the plug. But verify that too because you can be lean on the plug and dead rich in the chamber.

That doesn’t look lean from what I can see. You really need an otoscope to look into the shell of the plug where it meets the porcelain. There should be an .080-.100 wide fuel ring around the bottom of the porcelain.

What plug is that and what fuel. It looks a touch on the hot side.

Edit: why are you going through sensors?? One thing that will kill them is using silicone that isn’t O2 safe.

If it doesn’t say specifically it’s O2 safe it may not be O2 safe silicone.
 
Autolite 65, first tank this spring was BP unleaded premium. I'll run non oxygenated when it's available.
 
AEM 30-4110 UEGO controller. 1st 2 in exhaust sensor didn't work. 3rd one seems to work, not sold on the accuracy of it.
 
Ok, that model isn't as prone to ground issues and doesn't need calibration, so there are fewer user errors possible than with some models. And it's easier to have a false lean reading than false rich. So it could be a gauge issue, or it could be the #2 cylinder is lean. Check to be sure the plugs are even - but if you've had issues with two sensors already, the gauge and its wiring are the common factors. If you're using a chassis ground, try relocating the ground wire to the block or battery.
 
Yeah the chart assumes you are running petrol with no ethanol in it

i'd concentrate on moving idle up to average of about 13.7
aiming for 14 when i lift off the gas at speed
worry about the top end when you get the bottom end done

The stoichiometric number for e10 10% ethanol fuel is 14 to 14.1 and non ethanol fuel should be 14.7

mismatch between plugs is more likely distribution or ignition. if you have flat spots you will be leaving unburnt fuel to gum up your plugs

you might need your idle mix screws turned in or out so they don't match side for side depends on the carb and plenum size, how tall the carb mounting flange is and the firing order

the holes that the idle mixture comes out of favour the cylinders that draw it more directly
if it has to do a 180 turn it will be differently mixed and atomised so there will always be some mismatch, can't say if that is going to make a cylinder rich or lean because that is down to how well it burns as well as how much is delivered and how finely graded the fuel is in the air .
 
Don’t worry about reading the plug for idle, cruise, or transient throttle positions, you’ll just drive yourself crazy. Read the plug for WOT fuel and ignition. Get the gauge responding correctly (I have an AEM UEGO in my truck and it’s fantastic) and once it works you’ll learn to trust it. After the cruise and idle AFR are sorted out THEN work on WOT and read the plug, comparing it to the wide band. The emphasis should always be on the plug.
 
also remember.. only believe the gauge when:-

1) all cylinders are firing
2) you can pull smoothly through the rpm from idle to your desired rpm
3) there are no rich or lean flat spots

if there is a flat spot the rpm before it gives a better indication on the gauge of the direction you are going than the mess you see as it tries to recover afterwards

the sensor was made for a FI set up.... it expects the fuel control to be within the boundaries of an ECU i.e all cylinders firing and no flat spots

it does not work when the range of wrongness is a wide as what is possible with a carb and ignition timing based on springs and weights and position

i tune to 13.5 at WOT but on the way if it dips into the 12s i don't care i have the equivalent of 1 carb per cylinder so hours have been spent messing about with this stuff

car will run fine in the 11s at WOT.. without the gauge you would never know
at that RPM it will lap up what is thrown in, at least until you foul up the plugs.

Also at that rpm and load, any waste fuel is out the tail pipe it is not going to have a chance of wetting the cylinder walls. its too hot in there for that.

it will get hotter quicker and you will feel it as a "something not quite right" at 15 upwards

very useful tool , in a tool box of common sense some helpful pointers, a decent timing light and the right ignition curve for today (modern gas) and your state of tune (CR cam headers etc).

so i'm just passing on the helpful pointers i got off another bloke (some would say i have no common sense to pass on :) )

served me well and stopped me diving down rabbit holes for months at a time

get it more or less right
then use the gauge to really get it right

dave
 
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