Old School Meet New School: How to Get EFI-Quality Mileage From Your Carburetor

Think you can’t have a small-block that lays down 556 hp, 512 lb-ft of torque, 120-mph trap speeds, and 21 mpg on the highway without dropping three grand on a slick EFI setup? Think again.

The secret? Wideband tuning and adjust-able carburetor fuel circuits that allow you to tune idle/cruise mixture and curves. “Off-road use only” combinations of cam-shaft, cylinder heads, and quench area can become surprisingly docile with some attention to carburetor tuning. Most over-the-counter (OTC) carbs come with a tuning baseline that will run OK in the average car, but not exceptionally well with your specific application. With wideband tuning and careful attention to two key circuits found in most OTC carbs, you realize fuel mileage that rivals that of the latest EFI setup. Yes, having an overdrive to slow the final drive ratio down to the 2.5:1-2.6:1 range helps, but the mileage benefits of a 2,000-rpm cruise can be easily nixed by an untuned carb dumping a 10:1 air/fuel ratio into the motor when 15:1 is nice enough.

Most people think of tuning a carburetor via two circuits: the mains and the idle mixture. For this article, we’re going to go way deep on the idle circuit and ignore the mains/wide-open throttle. Contrary to popular belief, the mains don’t really come into play until engine speed reaches 2,500- 3,000 rpm and assumes a low-vacuum mode, like when your right foot is getting acquainted with the carpet.

The often-overlooked idle circuit determines the idle mix, as well as cruise and part-throttle efficiency. Get the idle air/fuel mix in the 14-15:1 range at idle, transition, and cruise, and you’re way ahead of a carburetor that dumps 10-11:1 buckets of gas through your motor. During light load conditions, your engine can get by fine on 14 to 15 parts of air for every part of fuel (6.8 percent of the mixture by volume), which equates to 33 percent less than an 11:1 air/fuel ratio

WHAT YOU’LL NEED:

* Innovate Motorsports LM-1 Wideband tuning device and LMA-3 Aux Box data acquisition (roughly $590)
* Idle-feed restrictor blanks ($14, Barry Grant PN 200083, drill to size)
* Air bleed blanks ($13 for Barry Grant PN 200082 or $26 for Holley 126-28-10, drill to size)
* Small drill bits, 0.018-0.100-inch ($20 per set from most hobby/remote control shops)
* Power drill and small bit chuck (to handle the very small drill bits)
* Narrow-blade standard screwdriver (for changing air or fuel bleeds)
* Line wrenches and Allen- or screwdrivers for removing fuel bowls
* New bowl and metering block gaskets (replace if old or questionable)
* Small snack bags (to hold IFRs or ABs separately)
* Sharpie permanent marker

BASELINE

First, make sure that the primary and secondary throttle blades uncover 0.020 inch of the transfer port. This is standard procedure with most Holley and Demon-style carburetors and is usually done before putting it on the manifold so that you can visually check and measure the amount of transfer port being uncovered.

On a four-corner idle circuit carb, you may be able to gain some idle speed by opening the secondary throttle-blade position a hair. If the engine still doesn’t have acceptable idle speed and quality, you may consider drilling a 3/32­inch (0.09375) hole in the throttle blades. Mighty Demons, such as the one used on our test car, offer an alternative to drilling the blades. You can increase the idle air allowed into the engine via its Idle-Eze adjustment found underneath the air-cleaner stud.

Next, gently rotate the idle-mixture screws clockwise (on both sides of the carb, or on all four corners if your carb is equipped with four-corner idle circuits) until they’re fully seated–don’t over-tighten. With all four idle-mixture screws seated at zero-air/fuel admission, back each one out 1.5 turns counter-clockwise. This will give you a base- line amount of air/fuel being admitted to the engine at idle and allow you to get the car started and running. After you start the car, you may have to adjust the idle-mixture screw outward (counter-clockwise) or inward (clockwise) by as much as a full turn to keep the engine idling.

Look at the Innovate LM1 readout at this time to check the air/fuel mixture. For now, aim for 13.5-14:1 to get the engine up to operating temperature. The air/fuel ratio will richen up as the engine reaches operating temp. Once it’s fully warmed again, adjust the four idle/cruise-mixture screws until you’re in the 14.0-14.4:1 range at idle. Ideally, you’ll also have the Innovate Aux Box set up so you can monitor engine vacuum, rpm, and acceleration (more on measuring acceleration later).

Using the LM1 wideband monitor and Aux Box data acquisition really makes this step simple, since you can maximize the screen on your laptop while the LM1 is attached, and adjust the idle/cruise-mixture screws (for air/ fuel) and Idle-Eze screw (for idle speed and air/fuel). The LM1 and Aux Box provide you with six exceptionally easy-to-read gauges, enabling you to tune a solid idle, a higher idle vacuum, and an air/fuel baseline of 14:1. The high-visibility dials (see close-up) make it easy to immediately see subtle changes to air/fuel ratio, idle rpm, and manifold vacuum in real time, while you’re tuning the car. Now check the vacuum reading at idle. Our hot-cam small-block (253 intake, 259 exhaust duration at 0.050 and 0.640-inch lift, 110-degree lobe separation) idled at 9-9.5 inches at 820 rpm. With this in mind, we installed a 6.5-inch power valve, allowing the engine to get a healthy splash of fuel when engine load (vacuum) dictated it. Since it’s only 3 inches below idle vacuum, this is a fairly loose power valve. We’re on the lean edge of idle/cruise while also delaying the onset of the mains. So having a power valve 3 inches below idle vacuum has proven to keep throttle response crisp without dumping in fuel. Most tuners would stop at this point and begin jetting the mains. But hold on–here’s where we improve fuel economy and provide razor-sharp throttle response.

ENHANCED TUNING

The first tool in the enhanced tuning section is the Idle Feed Restrictor (IFR), which is located in the metering block, so you’ll need to drain and pull the fuel bowls to get at it. If you have a four-corner idle/cruise carb, there are two diagonally oriented removable bleed/restrictors on the primary metering block and two on the secondary side. The IFR is the gatekeeper of fuel for the idle/cruise circuit. Experienced tuners decrease IFR size when they need to reduce idle/cruise below 1/2-3/4 turn from fully closed in order to get 14:1 air/fuel ratios at those speeds. The IFR is flooding the idle circuit with fuel and causing the idle and cruise to saturate. Alternatively, tuners can increase IFR size if the engine is starving for idle/cruise fuel (beyond 2.5 turns out from fully closed/seated idle/cruise screws to keep the engine running). Graphics always tell the story well, so here’s a look at Innovate’s Logworks data logging facility. As the car crests a hill, notice the blue manifold-vacuum line rising as the idle/cruise air/fuel mixture falls into the 11-12:1 range. Leaning out the IFR should help these lines move in parallel instead of opposite directions, as seen in this diagram.

Our test car yielded idle mixtures in the 13:1 range when there was very light load at cruise, but when the load changed, the ratio plunged to the 12s and 11s. Pig rich. With the stock/OTC 0.036-inch IFR in our 750-cfm Mighty Demon, we couldn’t out-mpg a Ford Excursion. To compensate, we dropped the IFR from 0.036 to 0.020. Since we’d cut so much fuel out of the circuit, even with the idle/cruise screws backed out from 1.5 to 3 turns, the engine would barely idle or run under 2,000 rpm. This meant we had succeeded in limiting fuel flow through the idle/cruise circuit. The next step was to change the rpm and rate at which the idle/cruise circuit allowed admittance to the new and thrifty metering curve.

We then began adjusting the second component in the idle/cruise circuit, the Idle Air Bleeds (IABs). IABs are small screw-in “air jets” that reside in each barrel of the carburetor–they are the ones farthest away from the fuel vents in the Mighty Demon, as shown in the accompanying diagram.

IABs function as a mini-carburetor within the idle/cruise circuit. They take the fuel from the IFR and bleed in a small amount of air under vacuum, before this air/fuel mix makes it to the last step into the engine (the Idle/cruise­mixture screws). The IABs allow you to tailor what vacuum/load and at what rpm the idle/cruise circuit will begin to meter air/fuel to the engine. With our engine starving for air/fuel, thanks to the ultra-lean IFRs we’d just installed, we had to make our leaner air/fuel curve come on-line earlier via a smaller IAB hole. The stock IABs are 0.070 inch; we went down to 0.032 inch. With the smaller IABs, the car fired right up and idled with the idle/cruise screws 1.6 turns out from seated. We set the air/fuel at 14.3:1 on a warmed-up idle, and the engine ran perfectly at 14.0-14.7:1 from 1,400 to 2,100 rpm (part throttle through the gears as well as cruise).

The next step was to rein in the mains. They were beginning to make the air/fuel mixture go rich under load (14 inches of vacuum at cruise or less) or above 2,500 rpm. The idle/cruise circuit was now doing its job flawlessly, but the main circuits were ready to join the party too early under those same conditions. Air/fuel would fall to high-11s and mid-12s at 2,300 rpm or under load.

Since we had just enabled the idle/cruise circuit to function from 900 through 2,900 rpm, we didn’t need the mains joining in at 2,400 rpm. We remedied the over-rich condition in the mid-range cruise by retarding the mains’ high-speed air bleed (HSAB) circuits from coming in as soon–the opposite of what we did with the IABs. Since the mains were allowing fuel to come in too early, we opened up the HSABs from the stock 0.039s to 0.093s. The jumbo 0.093-inch HSABs delayed the onset of the main circuits to 2,800 rpm, as shown in the green and blue bars in the accompanying charts.

Sure enough, the 0.093s kept our air/fuel in the low-14s all the way up to 2,600 rpm on level ground and helped keep it in the high-13s when pulling up hills or accelerating lightly. The chart below reveals our final state and illustrates our improvement in air/fuel at cruise from the mid- 13s to the mid-14 range, pulling 200 more rpm at cruise and the same vacuum. No stumbles and no hitches, other than the occasional tapping of the fuel gauge just to make sure it hadn’t frozen up. The trip back from the race track had us with traffic running 65-70 mph, and we saw 21 mpg when we filled up back at home.