Idle Mixture Tuning
One of the "thrills" of installing an Air Fuel meter on your carbureted Ford for the first time is seeing how far off your idle mixture screws have been all these years. Since time began you may have been going through the same routine of backing off your idle mixture screws as if you really knew what you were doing. Or, maybe you're car has been running just fine with the factory idle mixture preset. Whatever the case, the chances are once you get a good Air Fuel reading then you'll see that your carb can use at least some minor idle mixture adjustment.

Another interesting part of getting quantitative feedback from and Air Fuel meter is finally being able to equate what a quarter turn, half turn, or full turn of a Holley idle mixture screw actually does to Air Fuel ratio. Like the chart on the previous page, the graphic below is not a hard and fast rule but just what we experienced with this combination. Somewhat interesting was that our test vehicle's rich idle condition was cured with just a quarter turn of lean adjustment (clockwise).

The ACCEL DFI Wide Band controller is durable enough to be mounted in the motor compartment. We tied back the serial cable since we were just going off real-time gauge feedback.		As noted in the illustration above, once the entire unit was installed, the gauge indicated a rich idle condition with an Air Fuel ratio of 12.6:1. This photo was taken with the Torino in park idle.
Here we are searching for the correct lean adjustment to bring the idle Air Fuel ratio close to 14.2:1.		One quarter clockwise turn of each primary idle mixture screw brought us from 12.6:1 all the way to the recommended baseline of 14.2:1
There it is, we just set the idle mixture confidently and without the use of a laptop or data logging software.

Tuning for Cruise (power valve and primary jet adjustments)
With the easy idle adjustment out of the way, we looked to the primary metering circuit to dial in Air Fuel ratio at part throttle or cruise. Since we were originally running rich during all three driving conditions, we took on the power valve before making jet changes in order to confirm that the power valve was not coming on too early. If you have the incorrect power valve for your motor's vacuum and load behavior, then chasing a target Air Fuel ratio with jet changes is a fruitless venture.

A power valve is designed to supply additional fuel (as much as 10 jet sizes) during low vacuum and high load conditions, like transitioning from cruise to wide-open throttle. However, as indicated by the in-car video on the previous page, we suspected our power valve was coming on too late (low rated power valve) causing a lean condition during the transition from cruise to wide-open throttle. Take note of the slight hesitation and the short rise of Air Fuel ratio to 15.6:1 when the accelerator is fully depressed during the video.

For a street performance V8, a power valve that opens 2 inches (Hg) below the lowest steady cruise vacuum reading is most appropriate. A number 6.5, 7.5, or 8.5 power valve often works well on stock or slightly modified engines like our Torino's Cleveland. The lowest vacuum level we observed at cruise was between 8 and 9 inches (Hg). We cracked the primary bowl to see what we had, expecting to find a power valve rated below 6.0 inches (Hg). Here's what we found.

Sure enough, a 4.5 power valve was the culprit causing our lean-out from cruise to wide-open-throttle.

Based on the real-time feedback we received from our Air Fuel gauge and the vacuum reading observed during cruise conditions, we installed a 6.5 power valve.

This 650cfm Holley came equipped with number 68 jets in the primary.

Since we were running a tad rich during cruise (13.6:1), we replaced the number 68 jets with number 66 jets.

With a 6 inch (Hg) power valve rather than a 4.5 inch (Hg) our Air Fuel ratio did not rise above 12.7:1 when mashing the gas during cruise. Note that vacuum is at or near zero during this condition.

With the "lean" jet change we never dropped below 14.0:1 while cruising.

Tuning for Wide-Open-Throttle (secondary jet adjustments)
In our particular situation, the wide-open-throttle Air Fuel ratio was near optimum before any carb adjustments were made. Bouncing between 12.6:1 and 13.0:1 to start, the primary jet adjustment we made to lean our cruising Air Fuel ratio affected wide-open-throttle just enough to put us in a range between 12.8:1 and 13.2:1. If we found our Air Fuel ratio to be off by a full point or more during wide-open-throttle, the process of making secondary jet adjustments, up or down two at a time, would have been done.

Here's a shot a wide-open-throttle. The primary jet adjustment we made previously was enough to increase our average Air Fuel ratio at wide-open throttle to 13:0.1. No secondary jet adjustments were required, this time.

Conclusion
Ready to get yourself an Air Fuel meter now? We hope so. The intent of this article was to show that logical adjustments can be made to your Holley carburetor without trial and error tuning or worse, the fear of learning a new software program in order to sift through logged data. And don't worry, an Air Fuel gauge mounted in your carbureted Ford doesn't make you a tuner! Tell us what you think in the comments box below.