With our M715 sucking down fuel at a rate of 4 mpg, we knew we had to do some-thing. We were running a Holley 670cfm Truck Avenger on our Chevy V-8, and thanks to an AEM wide-band O2 sensor, we knew we were running an air-to-fuel ratio of 10:1 all the time.
The issue was that while we knew how to tune the idle circuit (little screws in the side of the carburetor) we didn't know how to get better mileage out of the Jeep without possibly damaging the engine. Better mileage means to suck down less fuel, but running the engine too lean will lead to damage. The old rule of thumb is that 14.7 units of air to one unit of fuel is the ideal, or stoichiometric, mixture for a gas engine to run. As we all know, in the real world, the ideal rarely happens and close is often good enough.
Such is the case with carburetor tuning. While a fuel-injected engine can monitor the air-to-fuel ratio (A/F ratio) and almost instantly change it by adding or subtracting fuel, it takes a little longer for the guys still running carburetors to adjust. How quick can you pop the hood to change jetting?
We knew that using a wide-band O2 sensor was a good way to keep tabs on what the engine is doing but we didn't know that it was such a great carburetor-tuning tool. When we went and talked to the experts at Westech Performance for some tips, tricks, and pointers and found them using a wide-band O2 sensor in conjunction with a dyno we knew we were on the right course. We strapped the truck to a dyno for a full carburetor tuning session and learned that if you don't have a good shop you can get your own carburetor pretty close to a full-on shop tune with a wide-band O2 sensor, a vacuum gauge and a bit of patience.
Even though we are tuning a Holley carburetor here, many of the tips can apply to whatever carburetor you might have.
More Power and Mileage
Tuning a carburetor is all about compromises and often you will see maximum power at a lower than "ideal" A/F ratio. In our case, we made maximum power below that 13.6:1 and 12.5:1, but we still made 15hp over where we started and still doubled fuel economy for our 7,000 lb truck. We went for economy, dial your carburetor in as you see fit.
Because of how great the company's...
Because of how great the company's parts are, we swapped an MSD distributor into the engine just prior to putting the truck on the dyno, but we didn't time it, we just tuned it by ear. As it turned out, we had almost 50 degrees of initial timing in this engine when we started and not only did that made the changes to the carburetor not have the desired results, but it screwed with the vacuum gauge we used later. It doesn't matter how good the distributor is unless you double-check initial, mechanically-advanced, and total timing before starting your carburetor tuning. It could save you lots of trouble in the long run. If you run at speed off-road you might want to try adjusting your accelerator pump rod and spring so that there is a little play to limit surging (arrow).
If you notice that you are...
If you notice that you are one point off the A/F ratio you want (12.5:1 instead of 13.5:1), step down four jet sizes. A good rule of thumb is that by going up four jet sizes will drop your A/F ratio one point and going down four sizes will raise it one point. Shoot for 13.6:1 on the primary jets and 12.5:1 or so on the secondaries to keep the engine from predetonating. This is the primary metering block, and the power valve is situated right between the two primary jets.
If you run a vacuum secondary...
If you run a vacuum secondary carburetor and you notice the engine falling on its face near the halfway point on the gas pedal, where the secondaries should open, check your spring. Too light of a spring can cause the butterfly to open too early before there is enough vacuum to draw fuel through the secondaries and too heavy of a spring can cause the butterfly to stay closed past the ability of the primary jets to deliver enough fuel.
Fuel is more dense than air,...
Fuel is more dense than air, so the air from the opening butterfly instantly gets sucked into your engine, but the fuel flowing through the primaries needs a bit of time to get going. The accelerator pump gives you an extra squirt of fuel right off idle to eliminate any dead spots from the butterfly opening. The squirter (shown) is basically a restrictor plate to the accelerator pump. Just like a jet, the squirter is sized. And, much like the jets, too small of a squirter can lead to a lean condition and too large a rich condition. Look for this just off idle.
One thing the vacuum gauge...
One thing the vacuum gauge is still good for is figuring out if you have the correct rated power valve. The power valve opens at wide open throttle when vacuum drops off and it is stamped as to what vacuum will open it. Run a manifold vacuum line inside the Jeep, and watch the A/F ratio gauge and the vacuum gauge at the same time (while someone else drives, of course). If the engine leans out before or after the valve's rating, change the valve. You should see a brief richening of the mixture near the rating of the power valve as it opens.