AFB Carburetor Reviews & Tuning Parts - Tuning The AFB

Carb tuning is a familiar procedure to old racers, something practiced from an early age, when anyone with a good ear and a screwdriver would put a tune to their wheels on a Saturday afternoon. These days, with OEM fuel injection systems having been the norm for a couple of decades, we've run across guys who have never even driven a carb-equipped car. Without good old learned experience, the art of carb tuning is filled with mystery. Really, there is quite a bit of flexibility in the metering design of a carb, and with a little tweaking in the right places, carb function can be tailored for the task at hand. We installed an Edelbrock four-barrel atop the tired 318 in our unrestored and ratty '68 Barracuda, and it worked well without any fiddling. Not content to leave well enough alone, we broke out our tuning kit and screwdrivers to fine-tune the carb for the application at hand. When we were done, we didn't find a miraculous boost in horsepower from our stock 318, but we definitely improved the efficiency and fuel economy with the razor-sharp tune. Let's look at the inner workings of an AFB-type carb and review the tuning parts and their functions.

Float Your Boat
There's no use trying to tune a carb if the basics are way out of whack. First, ensure that the floats are properly set. Carb floats are attached to the needle and seat valve and regulate the fuel level in the bowl. Floats set too high will cause the carb to run excessively rich, while having the floats too low will lean the mixture and potentially cause the carb to run out of fuel at wide-open throttle. The float adjustment on Edelbrock carbs is an easy two-step procedure. The air horn (or top of the carb) needs to be removed since the float adjustment is internal. Open the carb, and with the air horn inverted, gauge the distance between the top of the float and the air-horn gasket. Edelbrock gives a spec of 7/16-inch for float height. No special tools are required here; just use a 7/16-inch drill bit as a gauge. Bending the float arm adjusts the float height. Support the pivot end of the float as the adjustment is made to avoid pressure on the needle valve while bending the arm.

The second step is to adjust the float drop. The float has a tang on the pivot end that bears upon the seat to provide a positive stop. Measure the float drop with the air horn upright, and adjust the float drop by bending the stop tang. Float adjustments take only minutes and should be verified before any serious tuning or carb changes are made.

Idle Time
Setting up the idle is the most basic level of carb tuning. For the newbies, here's the simple procedure: Bench-set the idle mixture screws by screwing them in until they bottom, and then back each out approximately three turns. AFB idle-mixture needles work conventionally-out for rich, in for lean. With the engine running and warm, adjust the idle speed to the desired rpm, and then turn your attention to the mixture screws. The idea is to set the screws a bit rich, off (out from) lean-drop. With an open-plenum manifold, turn both screws in evenly a half turn at a time until the idle rpm drops noticeably. This is the lean-drop point. Then, back each screw out about 1/4-1/2 turn until the idle quality recovers, and it's done. With a divided-plenum manifold, the same technique can be used, but a final fine-tune can be made individually on each mixture screw using a similar technique.

If there is a significant upward rpm change as the idle mixture is zeroed-in, readjust the idle speed screw and double check the mixture screws to be just out from lean-drop. This technique uses rpm as a gauge of idle quality, which we gauge by ear, although some prefer to find lean-drop with a vacuum gauge or tach.

Primary Metering
The main metering system on an AFB is controlled by a jet and metering rod on the primary side. The jet is a fixed metering orifice, which regulates the fuel volume by acting as a restriction to fuel flow. To vary the open area of this orifice, and thus the mixture, the AFB carb utilizes a metering rod with two steps. Think of the jet as a hole for the fuel to flow through, and the metering rod as a stick in that hole. If the stick is thick, it will plug quite a bit of the flow area of the jet. The thinner it is, the more fuel can flow through. The AFB's metering rods are profiled with two steps, appropriately called the "lean step" and the "rich step," or sometimes "cruise" and "power."

Under higher vacuum conditions, vacuum acting on the metering rod piston pulls the metering rod down into the jet, aligning the thick "lean step" of the rod with the jet orifice. With the thick part of the rod in the jet, the fuel volume is reduced, corresponding to the reduced area of the jet open to fuel flow. Under higher load conditions, the engine's vacuum drops and reaches a point where it will no longer hold the metering rod vacuum piston against the metering rod spring's load. At this calibrated vacuum level, the spring will raise the metering rod/piston assembly, and the thinner end of the rod will now be aligned with the jet. With the restriction in the jet effectively reduced, more area is open to fuel flow, and the fuel mixture will become richer.

The foregoing description of the primary-fuel metering system shows that the primary-fuel metering is a result of the combined interplay of a few key components. All of these components-the jet, metering rod, and/or spring-can be altered to tune the fuel curve. The jet and the rod must be considered as a combination. First to consider is the WOT (Wide Open Throttle) mixture. At WOT, the metering rod will, of course, be operating on the thinner "power step" of the rod. Richening the mixture can be accomplished with either a larger jet or a rod with a thinner "power step." Rod changes alone can be made without disassembly of the carb, and can alter the mixture within the range of available metering rods. A jet change can be employed to make a larger change in primary mixture, particularly if the tuning process has exhausted the range possible with available metering rod diameters.

There are a tremendous number of combinations of jets and rods possible, and it's easy to get lost in the calibration process when considering the two variables. Quantitatively, the open area of a rod and jet combination can be calculated mathematically from the relative areas of the individual rod and jets being used. Altering the jet will also alter mixture in "cruise mode," while the engine is under higher vacuum conditions and the metering rod is down on the lean step. For instance, if the jet is increased to fatten up the WOT mixture, the cruise mixture will also become richer than the base setting. If the cruise-mixture ratio is to be retained, a metering rod that is thicker at the "lean step" must be substituted. Edelbrock has an excellent handbook, the Performer Series Carburetor Owner's Manual, which includes charts of the relative mixture changes from the base calibration with a wide range of rod and jet changes. For the tuner, this chart is an invaluable reference, graphically illustrating and quantifying the changes to both the "cruise" and "power" mixture for a given combination of rod and jet.

While the rod and jet control fuel flow, the metering rod spring controls the timing of the switch from cruise to power mode. A stiffer spring provides more tension against the vacuum attempting to lift the metering rod to the rich "power mode." Essentially, a stiff spring will open the richer fuel flow at a higher vacuum level, so the mixture goes rich sooner. Edelbrock has a spring assortment available to allow this transition point to be tuned. The base spring steps-up the metering at 5-inches Hg, and springs are available in steps from 3-8-inch Hg to alter the transition point. Tuning the spring tension is strictly related to driveability, since at WOT, any of the springs will have the same effect under near-zero vacuum conditions.

The spring comes into play if a flat spot (usually lean) is evident in the transition from light-throttle cruise to part-throttle acceleration. If such a flat spot exists in moderate part-throttle operation, checking the vacuum level with a vacuum gauge can help isolate the problem. Say, for instance, there is a flat spot occurring under moderate load at 6-7-inches vacuum, clearing up once the throttle is pushed a little harder, and the vacuum gauge reads below 5 inches of vacuum. If the stock 5-inch spring is still in place, it's safe to conclude that the flat spot is related to a lean mixture at the higher vacuum condition, resolving itself as the metering rod opens to the "power step" at 5 inches. Substituting a higher vacuum spring, such as one rated for 7 inches of vacuum, will likely provide the solution. As a rule of thumb, use the lowest vacuum-level spring that will provide smooth operation to prevent needless enrichment and loss of economy.

Secondary Metering
Calibrating the secondary side of the AFB-style carbs is simple in relation to the primary side. Charged only with providing mixture at high-throttle opening, a simple jet is all that is employed at this end of the carb. Tuning is simply a matter of swapping jets, the larger the richer, and the smaller providing a leaner mixture. Unlike the primary, where some range of adjustment is possible without disassembly of the carb via the metering rods, any change in the secondary jetting requires that the air horn be removed for a jet change. Changing secondary jets once the air horn is off is as simple as unscrewing the old ones and screwing in the new.

If a serious level of mixture calibration change is being performed on the carb, the primary and secondary should be adjusted to a similar, relative amount to arrive at the required final mixture setting. For instance, if the carb is found to respond to enrichment of the primary, the secondary should be enriched a like percentage. It's better to aim for an enrichment of 4 percent at both the primary and secondary circuits than to fatten the primary 8 percent and neglect the secondary. Edelbrock's handbook breaks down the percentage change for various jets and rods in their respective charts, providing an almost essential reference for the changes being made.

Pumping Possibilities
The accelerator pump is a key component of the carburetor in terms of response to sudden throttle application. If the pump's not right, there won't be any burning rubber or snappy response when hammering the "go" pedal. AFB carbs use the familiar displacement-piston accelerator pump, which channels raw fuel to the primary carb barrels. Displacement, as any engine guy can attest, is a combination of bore and stroke. The accelerator pump piston bore is fixed, but the pump has one adjustment, and that is for stroke length. The quantity of fuel available via the accelerator pump can be increased by increasing the pump stroke, and conversely, decreased by lessening the stroke. Changing the position of the linkage rod on the pump arm varies the stroke. The hole closest to the pivot point provides the greatest volume of fuel. It is important to note that the pump-arm position sets the linkage travel, and thereby the stroke. However, if the linkage rod is maladjusted or bent, the piston's actual stroke may not increase at all (if the start height isn't where it needs to be). A quick check can be made by holding the throttle wide open and checking that the piston does not bottom-out before the throttle reaches its open stop. If the piston bottoms prematurely, shorten the pump linkage by bending so full pump travel coincides with full throttle.

While the pump travel adjusts the volume of fuel available through the accelerator pump circuit, the discharge nozzles affect the pump-shot timing. Large nozzles dump the fuel volume quicker, but since the volume is fixed, a shorter pump duration will result. Conversely, smaller nozzles will deliver the same quantity of fuel but will meter it in more slowly. The required nozzle depends on the combination and is best determined by trial and error. Deep gears and race cars equipped with high-stall-converters generally favor a quick reception of the fuel shot. Discharge nozzles can be drilled, but, for different combo possibilities, assortments of various sized nozzle assemblies (clusters) are available from Edelbrock. The replacement of the pump cluster requires the removal of the air horn to gain access.

Tune Time
We recently installed an 800 cfm Edelbrock carb on our '68 Plymouth Barracuda's tired old 318. We found that the performance seemed good without any tweaking, actually overpowering the tires with a stab of the throttle. Amazingly, with such a preposterously oversized carb, we encountered no drivability problems. The 800 is Edelbrock's largest carb of this style, and our selection criteria certainly deserve some criticism. The 318 is dead stock, except for the duals attached to the factory exhaust manifolds and an antique Edelbrock Streetmaster intake bolted up top. A carb was absent when we purchased this jewel; however, we had the 800 carb handy, and it bolted up and got us down the road. Frankly, we were shocked by how well it worked. A 600 cfm carb would have been a better piece to use. We did find that the oversized carb was grossly rich for our poor little 318. We figured that jetting would allow the carb to work better in our misapplication and also serve to improve economy until we were ready to get serious about a suitable combination of parts.

Carb tuning can be done on the track, testing on a deserted open road, or, most appropriately, on a chassis dyno. We decided to take the beastly Barracuda to Westech for a spin on the chassis dyno and a tune to the carb. The only problem was that the little fish was garaged a good 200 miles from our favorite test facility, and we just stabbed in an 8 3/4-inch rear sporting 4.10 gears. The sensible decision would have been to trailer it, but that would have meant giving up the chance for a marathon shakedown run. We were drawn to the open road, unsure if the decision was adventurous, a disaster waiting to happen, or outright foolish. Leaving the tool kit behind and armed only with a cell phone and AAA card, we had the old Plymouth gassed up and on the highway at dawn. The lack of a heater core in the dead of winter was annoying, but more troublesome were the open windows in the icy air to ferry out the noxious fumes emitted through the tailpipe-omitted exhaust system. Three hours and three tanks of fuel later, we rolled the flat-black bomb into Westech, a little weary from the drive and the effects of a gut-full of carbon monoxide. The 318 fared better, buzzing for hours on end at around 4,000 rpm, quite an endurance test for a 35-year-old factory-built mill.

It seemed the ratty condition of our prized ride was an open invitation for abuse. Overworked from the high rpm jaunt, our trusty little 318's rear main seal was seeping oil on Westech's spotless floors, drawing jeers and comparisons to the Exxon Valdez. As the Barracuda was dutifully strapped to the SuperFlow chassis dyno, the air was filled with the sound of haggling and wagering over how many runs the engine would survive. Justifiably proud of the 318's endurance, we defended its honor and laid down the challenge, despite that it was our only transportation home. "Blow up? Go ahead and try. You're not dealing with a small-block Chevy here," John Baechtel heartlessly brutalized the 318, maniacally winding it well into valve float on its fragile and fatigued stock valvesprings. Ha! Nary a protest from the reliable old 318, except for the momentary lifter clatter at the end of the runs as the tappets regained their equilibrium. Tough, those 318s.

The baseline pulls showed an absurdly rich mixture, falling within the low 10:1 range on the dyno's readout. Leaning out in steps following the Edelbrock chart, we first went 4 percent lean on the primary and secondary. This was still not enough, so we followed the charts until we had dropped the fuel flow 12 percent, or three steps lean. The instru-mented fuel ratio read more appropriately in the mid-12s. Power wasn't up radically, but we didn't expect it to be. In fact, the 318 was making pretty good output, with nearly 200 hp showing at the wheels. That's 5.0 Mustang territory, and this was a high-mileage grandma mill, far past retirement age. It always did feel pretty spry.

The tune had one dramatic effect, and that was a significant increase in economy. We didn't tally the CAFE statistics, but we only had to make two stops on the 200-mile journey back in. What did all this prove? Once the tuning basics of the Edelbrock carb are understood, mastering the metering is no mysterious matter. Four hundred miles in this ragged roller was enough to gain some new respect for the venerable 318, although we've got to do something about that exhaust. Now, if you'll excuse us, we're headed for the oxygen tent.