[RocktimusPryme]

Had a post a while back. I was running lean on my wide band at cruise. Like low 15s. Turned my adjustable apt on the Q Jet a little and it was more in line with what I want. Like low 14s high 13s at cruise.

Drove the car to work the other day. Was fine on the way in. Drove it to lunch and back. Fine. Drove it home and now all of a sudden it’s in the 16s at cruise. Nothing else changed. No leaks that I can find. The regulator gauge shows 7.5 psi.

Drove it again today, more or less the same. 15-16:1 at cruise. Piece of trash in the carb?

When you stomp on it and the secondaries open it’s fine. 12 to 1

[Chief of the 60's]

Time for that Holley

[RocktimusPryme]

Originally Posted by Chief of the 60's

Time for that Holley

If I pull the Q jet off I probably go to injection. But I’m not sure it’s the carb.

[Chief of the 60's]

Injection is nice but I'd rather be..........

[Scott65]

Leaking a little vacuum somewhere? Lines, or canisters?

[ta man]

Can you feel the difference driving? Is the engine surging..how do your plugs look..?

[Skip Fix]

Main circuit bleeds can affect cruise AFRs. My '73 750 cfm Q jet on the 81 will get leaner AFR as cruise rpm increases. My Demon 850 does the same thing on the 78 but the Holley 850 does not-different bleed sizes.

[RocktimusPryme]

Originally Posted by ta man

Can you feel the difference driving? Is the engine surging..how do your plugs look..?

No you can’t feel the difference. Plugs looks normal after my first lean condition issue. I haven’t looked recently.

I will check vacuum lines. I think I only have 2. Distributor and pcv.

[JLMounce]

Is there a reason you're targeting such a rich cruise mixture? At part throttle cruise on level ground, you really want to be in the mid to high 14 range, even into 15:1 at low loads it typically fine.

[Tom Vaught]

What fuel are you running thru the engine?

You might want to read this link vs go solely by a Air/Fuel Reading on a EGO Sensor.
(I help people with this all the time when they are trying to calibrate a carb.

http://ultra-gauge.com/customer_supp...php?article=29

Some of the info.

Air to Fuel Ratio (AFR)

Occasionally we are asked if UltraGauge supports real-time AFR. We have in the past not supported it. Note that it is supported by the UltraGauge MX and Blue as a user programmable parameter, but not recommended. Please read on to understand why.

Real-time AFR can be determined on those vehicles which support wideband O2 sensors. Wideband O2 sensors are less common, but very much superior to the older narrow band O2 sensors. Narrow Band O2 sensors have a very narrow range of useable operation around the Stoichiometric ratio. The Stoichiometric ratio is the point at which there is just the exact amount of Oxygen to burn the available fuel. The narrow band sensors essentially can only tell the ECM that the mixture is lean or that it’s rich, but not the degree. In fact if you monitor the O2 sensor output, it constantly switches from lean to rich, and rich to lean, as the ECM attempts to keep the Air to Fuel mixture at the Stoichiometric point.

Wideband O2 sensors have a much broader linear range of operation and if monitored generally provide a relatively constant output corresponding to the amount of oxygen in the exhaust. As a result, wideband O2 sensors can be used to provide real time AFR. The ECM monitors the wideband O2 sensor and outputs the ratio Lambda.

Lambda = Actual AFR / Stoichiometric AFR.

When the AFR is ideal, Lambda is 1. When the mixture is Rich, actual AFR is reduced and Lambda is less than 1.

If the Stoichiometric AFR is known for the fuel in use, then the Actual AFR can be determined

Actual AFR = Lambda * Stoichiometric AFR = (Actual AFR / Stoichiometric AFR) * Stoichiometric AFR

But here in lies the problem. The Stoichiometric AFR is never known because the makeup of the fuel that comes from the pump is not known. For example, this table provides the Stoichiometric AFR for various ideal fuels.


Fuel


Stoichiometric AFR

Pure Gasoline = 14.7:1

10% Ethanol Gas = 14.04:1

15% Ethanol Gas = 13.79:1

E85 = 9.75:1

Pure Ethanol = 9:1

Diesel = 14.6:1*

The problem is that pure gas is never pure, and a 10% blend is rarely 10%. That's why the pumps reads: "May contain 10%...". But in reality, it could be 1% or 15%, or any percentage in between.



Without knowing the Stoichiometric AFR for the fuel in your tank, there is no way to use wideband O2 sensor and lambda to determine exact value of AFR. Most AFR meters simply assume pure gasoline and use a value of 14.7:1. However, the O2 sensor cares little that you are using pure gas or pure Ethanol. For both it will report a Lamda of 1.

So let's say you have E85 in the tank. What will your AFR meter read? It will read 14.7:1, because Lambda is 1. But we know the AFR should be around 9.75:1. This is why reporting AFR can be so misleading and absolutely wrong.

The far better parameter to monitor is Lambda, as Lambda is independent of the fuel used. As long as Lambda is very near or equal to 1, you know your mixture is correct (Stoichiometric). If for performance reasons, you still wish to monitor AFR, because you wish to run rich, Lambda is still the better parameter to monitor as AFR will be distorted by the Stoichiometric AFR assumed. Using the MX or Blue, you can program any Stoichiometric AFR you wish, but it is still best to simply use Lambda.

Tom V.

(I did EFI calibrating for Boosted engines for a period of time).
One of the guys I was helping had a dual turbo BB Chevy with staged turbos. Big Turbo feeding into a smaller one. HIGH BOOST.

He used a GM Fuel Sensor set-up he modified to read actual pump fuel and what the fuel number Gasoline vs Ethanol was.
The new vehicles (Flex Fuel vehicles) do this automatically.

[RocktimusPryme]

Originally Posted by JLMounce

Is there a reason you're targeting such a rich cruise mixture? At part throttle cruise on level ground, you really want to be in the mid to high 14 range, even into 15:1 at low loads it typically fine.

Just piece of mind. Right now it is occasionally dipping into the red on the gauge.


And Tom, one thing I didnt think about. I got gas while at lunch that day when it started. Normally I run 93 that has the up to 10% ethanol. This station didnt have it. But they did have non ethanol 90. So i tried that. I wouldnt think that would be enough to be a point and a half off on the gauge, but it is interesting that it started right after that tank of gas.

[RocktimusPryme]

Went outside, and worked on it some. I moved the ATP a little just to take a drive and see the reaction. While under the hood I noticed that the distributor cap had one of the tangs come loose. Seemed on straight still, but I put the tang down and took a drive. It was cruising in the 12 now. So idk, maybe the cap was just askew enough to get a weak spark in a couple cylinders. Maybe its an intermittent issue and the cap was coincidence. Not sure at this point.