[john65ss]

I've got a late single-main-air-bleed carb with air bleeds right around 0.050", and a 50M / 73 rod/jet combination. I've got the APT screwed all the way down and the carb runs great everywhere (light throttle, heavy throttle - primaries only, etc). Should leave well enough alone, However....

Since the APT is screwed all the way down, I can't go any leaner with light throttle cruise. I was wondering if I should maybe try swapping in some 52M rods and dialing it down to the point of lean surge and then backing off in small steps to get to the leanest possible setting. I just wonder if I am leaving a few MPG on the table with my current combo.

Any thoughts? Should the "right" metering rod allow you to crank down the APT to make it "too lean"? With the current set-up, APT screwed down, I can tip the choke blade in a bit at 2200 rpm and get a 50-100 rpm increase, so I think I must be pretty close to a good cruise mixture. I just wonder if a larger rod might let me dial it in even closer.

[Cliff R]

The only way to answer that question is to try it.

Lean mixtures aren't always the answer to improved fuel economy as there is a point of no return with these things. At least playing around with the APT only effect light part throttle, so no potential engine damage will occur from the testing.

I've never found going really lean to help much with MPG's, unless you do ALL the driving like there is an egg under the pedal and you don't want to crush it.

Timing added by the vacuum advance is a player here as well, so it may take a few more degrees timing in conjunction with the leaner APT settings to see full benefits.

There are a few other "players" at work with these things as well, thermostat temperature is one of them. A hotter thermostat will help heat up the engine parts and make it easier to effectively burn a slightly leaner mixtures. Spark plug heat range is another, as is the plug gap with any spark plug being used.

I've probably spent 100,000 hours testing in that area with different q-jet equipped vehicles I've owned over the years, looking for perfection with each combination. Pretty fun stuff, and a lot to learn in that area as well.

Let us know how it turns out?.......Cliff

[77 TRASHCAN]

What kind of MPG do you get now???

Your MPG can only be SO good. You can't cheat physics, It takes a specific amount of fuel and air (properly mixed) to move a vehicle in either the MPG or extreme modes...Race'n and street driving are two different animals, although both still animals...

I got 22 MPG w/ my T/A in it's early days of driving, 21 MPG was easy and 20 MPG was real easy. Mind you 2.41 gear and 55 MPH speed limits (I usually drove 65 though).

How many cubes is engine???

Glad to hear your combo IS working...

[TransAm525]

As mentioned, leaner isn't always better. For maximum power, the mixture should be just slightly rich of stoich (14.7:1 air/fuel ratio), and for maximum economy slightly lean of stoich. If you keep leaning the mixture, your HC levels will increase (and your eyes start burning) as the cylinders start cooling and the air/fuel mixture isn't fully burned. If you run the mixture too rich, the cylinder temperatures will also drop, the plugs will begin to foul, and you may notice black soot emitting from the exhaust pipes.

Take a look in the archives, as Cliff and others have posted plenty of info regarding APT and Quadrajet tuning in general with basic tools. For precise tuning, install a wide-band oxygen sensor system with an air/fuel ratio guage/reader and go from there. If you're really serious about tuning for fuel economy, have some cash to burn, and have a set of headers installed, you can install an exhaust gas temperature system on your existing headers connected to a pyrometer and tune off the exhaust temperature (just like that of a reciprocating aircraft engine), however for nearly the same cost you can install a basic fuel injection system. Good luck.

[Cliff R]

Wide-ban 02 sensors are OK, but not needed for fine tuning the APT system. You still have to drive the vehicle and make small changes, record the results, and find the optimum settings for your application.

I always do ALL of my fine tuning first, idle, light part throttle via the APT, heavy part throttle, full throttle (drag strip runs), then I might hook up a wide ban sensor to see where I am at. The point of tuning is to find the best settings in all areas, not to chase a specific number down for every level of tune.

What I see a LOT in this hobby, is for folks to chase the tune to death with wide-ban oxygen sensors looking for specific results, rather than listening to what the vehicle is telling you in terms of performance and fuel efficiency at every level.

Folks also forget about ignition timing, it is equally as important when fine tuning part throttle A/F as the fuel curve. Adding timing helps reduce throttle angle and more effectively burns a leaner mixture. There is ALWAYS a "sweet spot" for any of these engines when it comes to adding a little timing at light part throttle via the vacuum advance.

Low compression engines will typically want more timing than higher compression engines, but cam choice does play a role in that deal, so some time should be spent in that area in conjunction with APT adjustments......Cliff