There appears to be a great disparity in the type of carburetor modifications performed by racers and performance enthusiasts; there is also an equal disparity in the degree of success achieved through those modifications. It would seem that Murphy’s Laws (anything that can go wrong will go wrong, just because you’re paranoid doesn’t mean they aren’t after you and countless others) are invoked whenever a carburetor is disassembled. And more often than not, simple misunderstanding is the culprit. You must comprehend the “whys” of a modification along with the “hows” if you are to experience the thrill of victory, rather than the agony of defeat.
For openers, the original carburetor application dictates that calibrations fall within certain pre-determined parameters. And since each carburetor circuit interacts with others, a simple modification can cause untoward ramifications, with the end result being a carburetor that suffers from “terminal dysfunction.” Whether you’re attempting to modify a carburetor to achieve improved performance, better drivability, or greater fuel economy, it is essential that you understand original design philosophies if you expect to achieve the desired results. Also note that radical alterations should never be necessary for anything other than highly specialized racing applications. The need for extensive rework is usually indicative of a damaged carburetor or misapplication.
Primary Idle System
Most carburetors, including the AFB and AVS, meter idle fuel through the same type of circuit—one that incorporates adjusting screws with needles that protrude into the curb idle discharge port. These screws are typically rotated clockwise to lean the idle mixture and counter-clockwise to richen it. However, some AFB and AVS carbs were produced with a single mixture adjustment screw controlling the flow through both primary barrels. Under this arrangement, bleed air, rather than air/fuel emulsion, is adjusted when the screw is rotated, but mixture is varied according to the same procedure used with conventional idle systems. When a single adjustment screw is used, it is left-hand threaded so that rotating it clockwise leans the mixture. This occurs because clockwise rotation of a left-handed screw causes it to back out, or loosen. Carter carburetors fitted with air bleed idle mixture adjustment also have fuel emulsion mixture needles, but these are sealed at the factory to limit the amount of fuel admitted during idle. Inspiration for this arrangement was provided by the federal government through its exhaust emissions regulations. Some original equipment carburetors were restricted even further, with all adjustments sealed.
Modifications: When and Where?
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In general, there should be no need for modification of the idle system other than adjustment of mixture and engine speed. However, if after a full range of adjustments are made and idle mixture is too rich or too lean, either the idle air bleed or the idle feed restriction has to be modified. (This isn’t meant to imply that these modifications must be attempted, but that they are the only options available other than replacing the carb.) Since the idle feed restrictions (also called low-speed or idle jets) on Carter and Edelbrock carburetors are usually placed in the main well, they are all but inaccessible and all idle calibrations must be made by altering the complementary air bleed diameters.
Determination of air bleed size is based on the diameter of the curb idle port, desired throttle plate position at idle, and transfer circuitry. Since all of these factors affect system timing, alteration of bleed diameter is not as straightforward as changing fuel jets. Consequently, air bleed modifications are generally ill-advised. If you’re not in the habit of heeding advice, ill or otherwise, keep in mind that drilling a larger hole is infinitely easier than drilling a smaller one. Another piece of advice: If a carburetor does not provide proper or even reasonable idle characteristics with factory calibrations, it is probably defective or is being used in an improper application.
The apparent need for idle circuit recalibration arises most frequently when a carburetor originally intended for racing usage is installed on a street-driven vehicle. Race carburetors are calibrated to provide a rich idle mixture as a means of compensating for exhaust dilution of the intake charge (a result of high cam overlap), and the relatively low fuel-metering signal that results from extremely long camshaft duration. When installed on a meek, mild-mannered street engine, a carburetor with race calibrations may feed excessive amounts of fuel at low speeds. This not only affects drivability but seriously degrades fuel economy, hence the need to lean the mixture. However, while enlargement of air-bleed diameter provides a leaner emulsion, this change also impacts circuit timing (the point at which idle fuel flows), and if too much of the signal is bled off, the idle circuit may stop delivering fuel before the main circuit begins. This can result in an off-idle stumble that causes the car to break into an automotive version of the foxtrot.
If you are prepared for the consequences and there is no alternative but to drill the bleeds larger, measure the existing diameter before you begin. If too much material is drilled away, epoxy can be used to seal the air bleed so that it can subsequently be redrilled to the original diameter—or to the size that seemed to work so well when you decided to “give it just a little more to be sure.”
Adjustable Air Bleed Idle Systems
As previously mentioned, some original-equipment carburetors used an adjustable air bleed to permit idle mixture alterations. These carburetors also contained a standard air bleed, but modifying its diameter serves no purpose, since this is effectively what occurs when the mixture screw is turned. When carburetors with adjustable air bleeds suffer from idle fuel delivery problems, it is because of too little fuel, not too much. The problem is complicated by later-model carburetors with the mixture screws sealed to prevent alteration of fuel flow and exhaust emission by “monkeying around” under the hood.
A common cause of extremely lean idle mixtures is the accumulation of deposits along internal fuel passages. This restriction can limit fuel flow to the point where correct idle speed cannot be maintained. If the deposits or dirt cannot be removed by immersing the primary castings (completely disassembled) in carb cleaner (with subsequent blasts of compressed air), chances are that adjusting the mixture screws will be to no avail as the blockage will remain. This continues to be the limiting factor in fuel flow. Always make sure that internal passages are clear of dreaded “foreign matter” before reassembling the carburetor; use thorough visual inspection and air-nozzle tests.
All tests and rebuilding aside, the best way to prevent a “smog-lean” idle is to install an appropriate performance-model carburetor—one that is designed for wide adjustments in idle-fuel ratios.
Main Metering System
In theory, no changes in the main metering circuit, other than jetting, should be necessary if the right carburetor has been chosen for a particular application. However, in some highly specialized race environments, it may be advantageous to alter the booster venturis or venturi diameter in an effort to gain a competitive advantage. Unfortunately, there is little magic to be had for the price of a screwdriver and a drill bit. And without a fully equipped airflow bench to determine fuel delivery curves, a main metering circuit modification is usually tantamount to a shot in the dark. This isn’t to imply that improvements are impossible, but a systematic approach backed by accurate record keeping is essential. The real problem with main metering system modifications is that they are not easily corrected, so if a mistake is made, the result will probably be the creation of a carburetor that functions better as a paperweight than a fuel handling device. To put things in perspective, consider the certain modification and effect relationships (see table 5-1).
Depending upon the particular model, it may or may not be practical to perform any of the above modifications. The important and simple fact to remember is that Carter and Edelbrock carburetors do not lend themselves well to major alterations. The AFB and AVS have removable booster venturis, but replacements are not sold separately, so the only option available is to modify existing components. Removal of material from inside and outside diameters of the boosters will allow only a slight increase in airflow and is usually not worth the effort. However, if the need for greater airflow is vital, the venturi itself may be ground and reshaped. This may only be done on AFB and AVS models, and only on the primary side of the latter since the secondaries of the AVS are straight bores with no venturi shape (the secondary bores can be enlarged and a new air-valve fabricated for the enlarged shape, but the project requires expert-machinist talents and usually results in only minimal improvements in airflow). On the Thermo-Quad, the booster is an integral part of the air horn casting, and when it is removed, the venturi (in the phenolic main body) is easily accessible. However, due to the nature of the material (it cracks easily), and the lack of room around the venturi, you’re advised to leave this area alone.
Table 5-1: Modification & Effect Relationships
Increase diameter = Richer mixture
Decrease diameter = Leaner mixture
Air Bleed Size
Increase diameter = Leaner mixture and delayed activation of fuel circuit
(Fuel metering signal strength is decreased)
Decrease diameter = Richer mixture and quicker activation of fuel circuit
(Fuel metering signal strength is increased)
Increased diameter = Reduces restriction, increases maximum airflow and reduces signal strength
Decreased diameter = Increases restriction, decreases maximum airflow and increases signal strength
Jets and Metering Rods
While the foregoing may seem out of context in a chapter devoted to modifications, it is intended as an accurate appraisal of the gains to be had and the problems you’ll encounter when ambitious alterations are attempted. As opposed to some carburetors that are modular in construction, Carter and Edelbrock carbs generally utilize only two or three major castings, and consequently do not lend themselves well to major modifications. Jet and metering rod changes are the most common (and the most successful) form of main-metering system modification. Carter used to offer Strip Kits for the AFB, AVS, and Thermo-Quad models. These contained a selection of metering rods, jets, and inlet needle/seat assemblies packaged in an easy-to-carry and see-through plastic box.
Strip Kits may still be found at swap meets and online auction sites, but they’re relatively rare. If you can’t locate an appropriate kit, you may be able to purchase the necessary individual components through an Edelbrock dealer. Edelbrock offers a variety of tuning parts including jets, metering rods, step-up springs (which control metering rod position), accelerator pump nozzles, high flow and off-road needle-and-seat assemblies, and gaskets. Calibration Kits, which include an assortment of metering rods and jets, are also available. In any event, it’s always better to avoid modifying internal carburetor components if possible. It isn’t without justification that modified parts have a reputation for delivering unpredictable performance results that are almost impossible to duplicate (and duplication is required if you desire to match the right and left side air/fuel ratios). Jets are best left undrilled as a rough finish on the orifice, or a reduction of the entry or exit chamfers can sometimes reduce—rather than increase—flow capacity, much to the confusion of the tuner.
Spare the Rod, Spoil the Carb?
In a sense, this is a very real possibility. The problem with rejetting any carburetor equipped with metering rods is that both cruise and power calibrations are affected. (This is not the case with carburetors that utilize a power valve because when the power valve opens, power-enrichment fuel flows through a separate circuit, not through the main jets.) In Carter, Edelbrock, and any other carburetor equipped with metering rods, all fuel flows through the main jets (except for idle circuit fuel). Consequently, if main jets are enlarged simply to provide a richer mixture for heavy-load/full-throttle use, the not-so-desirable enrichment of cruise air/fuel ratio will also result. In Carter and Edelbrock carbs, this pertains only to the primary circuit, since metering rods are not used in the secondaries. And if you’re paying attention, you know that the secondary throttles are closed under cruise conditions, so they have little—if any—effect on fuel economy during steady state driving conditions.
Since it is the area, rather than the diameter of the jet, and the restriction (i.e., the metering rod) that must be considered when computing the effect that a change makes on air/fuel ratio, a series of mathematical computations must be completed in order to accurately determine the effects of a jet change on both power and cruise mixtures. With your trusty calculator or computer at your side, you must first compute the area of the existing jet and the area of both steps of the metering rod. Then you have to subtract the latter from the former to determine the effective jet area at each step in the metering rod movement. After you select a different size jet, you have to calculate the new jet area and subtract the metering rod areas to arrive at the new effective areas. The differences between the old and new effective areas would then determine the percentage change in air/fuel ratio.
For example, consider an AFB equipped with 0.098-inch diameter primary jets, a metering rod with an economy step of 0.0745-inch, and a power step of 0.047-inch. Using the standard formula for area:
Jet Area = Pi × (Jet Diameter/2)2
Jet Area = 3.1416 × (0.098/2)2
Jet Area = 3.1416 × (0.490)2
Jet Area = 3.1416 × 0.2401 Jet Area = 0.00754 square inch
Applying the same formula to the metering rod diameters of 0.0745 and 0.0470 inch:
Metering Rod Area = 3.1416 × (0.0745/2)2
Metering Rod Area = 0.00436 square inch
Metering Rod Area = 3.1416 × (0.0470/2)2
Metering Rod Area = 0.00174 square inch
Now, subtracting the metering rod areas from the jet areas (0.00436 and 0.00174 from 0.00754) yields effective areas of 0.00318 and 0.00580 square inch for economy and power steps, respectively.
With the equivalent areas of the power and economy steps known, we have a baseline from which changes can be made. For example, if you determined that the power mixture should be enriched by 5% but the economy step should remain unchanged, then you would select a metering rod with a smaller power step (smaller than 0.047-inch). But, a metering rod with the specific dimensions you’re looking for may not be available. There are two ways to overcome this obstacle: (1) install a larger jet and metering rod that reestablishes the original rod-to-jet area relationship for economy/cruise operation (that is, a larger jet diameter in combination with a metering rod that has an economy step yielding the same 0.00318 square inch); or (2) you can machine the power-step end of the metering rod. This can be done by either twisting the rod between two surfaces of fine emery paper or mounting the rod in a drill press while you hold the end against emery paper or a flat file. (Be gentle with file pressure—if you apply too much, you’ll bend the metering rod.) If you choose to modify the metering rod, use a precision micrometer or caliper to determine that both metering rods (left and right side) have the same diameters and are correctly sized over the length of the power step.
The theory of calculating the new metering rod diameter or jet size from the desired step area can be explained most clearly by following through an example of situation (2) from the previous paragraph. In this example, Charlie the Carb Tuner wants a 5% richer mixture. (This equates to a 5% increase in the fuel-flow area when the metering rod is raised for power enrichment.) Charlie calculates this 5% increase in area by multiplying the original area by 1.05 (a 10% increase would require a factor of 1.10); in this case 0.00580 square inch × 1.05 = 0.00609 square inch—an enlargement of 0.00029 square inch (0.00609–0.00580). Therefore if Charlie reduces the metering rod area by 0.00029 square inch on the power step, the fuel-flow area at the power step will increase by 5%, producing the desired richer mixture. Since the original metering rod power step area is 0.00174 square inch and a reduction of 0.00029 square inch is required, the new step area must be 0.00145 square inch.
Now that Charlie knows the new metering rod area, he can calculate the new desired diameter that by using the following equation, which derives diameter when area is known:
When Charlie plugs the numbers into his calculator or math computation website, he gets the solution:
So a rod size of 0.043 inch—or a 0.004-inch reduction in the power step—will produce a 5% increase in fuel flow.
A Matter of Timing
In Carter and Edelbrock carburetors, power enrichment is activated by a piston/step-up spring assembly that raises the rod when manifold vacuum drops. When manifold vacuum increases to a level sufficient to overcome spring pressure, it draws the piston and consequently the metering rod downward, thereby reducing fuel flow. For AFB and AVS carburetors, Carter offered a piston spring assortment under part number 61P-1083U. The load ratings of these springs are 6, 7-1/2, and 9 ounces, which tells little about the effect on enrichment timing except that the weaker springs will require less force to overcome their pressure, hence they will lift the metering rods (enriching the air/fuel ratio) at a lower manifold vacuum reading. Looking at it with reverse logic and using the 6-ounce spring as an example, only 5 or 6 inches of vacuum (rather than 7 to 10 for the other springs) will compress the weaker spring, thereby “closing” the power-enrichment circuit (leaning out the air/fuel ratio for economy operation).
Since there is little reliable data on the correlation between the spring load and engine vacuum, the trial-and-error testing method must be used with Carter springs. The better choice is to use the springs in Edelbrock’s kit (part number 1464) which are color coded according to the manifold vacuum level at which they operate. Each pair of springs in this assortment is calibrated to require 3, 4, 5, 7, and 8 inches of Mercury as the point at which they’ll cause a metering rod transition (from up-to-down, or down-to-up, depending on whether manifold vacuum is increasing or decreasing).
In addition to changing springs, it is also possible to alter pressure by adding a shim, which effectively stiffens the spring. Removing a few coils has the opposite effect. Since Carter never offered a spring kit for the Thermo-Quad, adding a shim was and is the only available approach. However, note that the TQ has a three-step metering rod, so the effect of spring changes on the intermediate step mixture (part-throttle load) is an additional consideration.
The concept of tailoring the metering rod timing is rather straightforward, but it is truly amazing that both backyard tuners and allegedly competent racecar mechanics set about changing power enrichment timing points without the faintest notion of the manifold vacuum characteristics of the engine under test. The reason for such misunderstanding often arises from the simple lack of a vacuum gauge. Such a device is the only means of peering into the internal operating environment of the engine. A reliable vacuum gauge is absolutely essential when dealing with a metering rod equipped carburetor. Alteration of power circuit timing is a trial-and-error effort and reliable reference points can only be determined if an accurate vacuum or manifold absolute pressure gauge is connected.
Discretion: The Better Part of Valor
Selection of power circuit opening point should be based on the amount of manifold vacuum present during “normal” operation. Obviously “normal” covers quite a broad area, depending upon engine application. Where a large-displacement engine in a lightweight street vehicle may cruise at a vacuum reading of 18-in/Hg and rarely pull the needle below 6-in/Hg (except during full-throttle acceleration), an RV or motor home will be lucky to cruise at 9-in/Hg and will pull 18-in/Hg only when rolling down a long grade loaded with kids, in-laws, dogs, toys and a port-a-potty. With such a wide range of potential operating conditions, there is no “right” or “wrong” setting for the power-circuit opening point; it simply must be correlated to engine requirements.
Unfortunately, the bottom line is: In terms of “major” performance gains, fussing with metering rod timing may be more trouble than it’s worth. However, in cases where an engine is mounted in a heavy chassis or a vehicle used for towing, bringing the power circuit into play at a higher manifold vacuum reading can reduce or eliminate part-throttle knock. Quite a different situation exists with a high-performance or race engine equipped with a long-duration cam. Manifold vacuum at idle may be so low that the power enrichment circuit is always open (i.e., the metering rods are never pulled into the economy position). In this situation, reducing the metering-rod spring pressure allows the rods to return to the economy position. The low-speed circuit consequently provides a substantially leaner mixture and probably greatly improved part-throttle engine response and gas mileage. In these cases, alteration of metering-rod timing can have a significant effect, but otherwise it’s pretty much a matter of fine-tuning, and the effort required may not be justified by the benefits received. It all depends upon how far the standard setting misses the mark.
In theory, the metering rods should rise to the power step when the manifold vacuum is just below the lowest reading encountered within the normal cruise/light-acceleration range. Viewing the power circuit as an “extra gas switch,” the switch should be turned on whenever the engine is placed in a demand-for-power situation. If the opening point is too high, extra fuel will be added during the momentary vacuum drops that occur during light acceleration, needlessly reducing economy.
However, the “ideal” is not the whole story. The power enrichment circuit is sometimes opened early; i.e., at a higher vacuum reading than is found during high-load situations, to get an additional supply of gas moving. This “tuning” is performed more in response to an anticipated—rather than current—engine demand. Consider the characteristics of a typical V-8 engine. At idle, with no load applied, manifold vacuum will read in the 16- to 18-in/Hg range. During normal driving conditions, the reading will vary from approximately 9 in/Hg under light acceleration to 17 in/Hg at cruise. Only during faster-than-normal acceleration, as is encountered in speed contests, taxi cab rides, or driver “over-enthusiasm,” will the vacuum drop below 9 in/Hg. Therefore, the typical street engine spends most of its life in the 9- to 17-in/Hg range. For this particular engine/chassis combination, the following parameters exist:
1.) A 10 to 11 inches of vacuum “opening” point would richen the mixture unnecessarily during normal acceleration, causing a deterioration in fuel economy.2.) Timing the metering rods to raise at 6 to 7 in/Hg would leave a fuel delivery “hole” with no mixture enrichment between the high-load point at 6 in/Hg and the transition to the above 9-in/Hg cruise range. This delay in power enrichment can create stumble, misfire, and ping.
3.) Considering the above, activation of the power circuit at 9 in/Hg would be nearly ideal, since it initiates additional fuel flow precisely at the point where the engine is beginning to require a richer mixture.
As in most things automotive, trial-and-error experimentation proves to be the final acid test. Unfortunately, with the seemingly ever-changing characteristics of pump gasoline, repetitive experimentation may be required to maintain optimum performance and economy over the long term. In many instances it is necessary to initiate power enrichment before the optimum vacuum reading in order to prevent ping under partial load. While the engine might not be laboring noticeably, the load associated with 8 to 10 inches of vacuum is frequently sufficient to launch a normally respectable gasoline engine into a cheap impersonation of a diesel.
It is for precisely this reason that the three-step metering rod was developed. Extra fuel volume is admitted by the intermediate step and fills the need for a slightly-richer-than-normal mixture when the engine is under light load. With two stages of enrichment, partial and full, a lean cruise mixture can be maintained while surging, stumbling, and misfire under light load are eliminated. The three-step rod will not affect or even slightly enhance gas mileage. By bringing about partial enrichment at 9 to 12 inches of manifold vacuum, it is often possible to reduce main jet size (as compared to the requirement for an equivalent carburetor with single-stage enrichment) while maintaining acceptable drivability.
Tuning the Accelerator Pump
Carter and Edelbrock carbs use a rod and link arrangement to actuate the accelerator pump. This system can be “tuned” to alter pump characteristics by bending the rod or reinstalling it in a different hold in either the upper or lower actuation levers.
Discharge volume is controlled by the piston at-rest position (the position of the accelerator pump in the pump bore when the throttle is closed). As the at-rest position is raised, more fuel flows into the pump chamber below the piston. When the throttle is fully opened, the piston discharges the full volume of fuel within the pump chamber. However, if the piston is raised above the fuel level in the float bowl, a delay in discharge will result since an air gap is introduced between the piston and the fluid in the pump chamber. For this reason, float-level setting is important, as a low fuel level can result in stumble during acceleration, caused by a “leaner” mixture of air-diluted fuel in the pump shot.
For optimum performance and economy—with fuel level properly set—the accelerator pump piston should be positioned to discharge just enough fuel to provide smooth acceleration. An over-generous pump squirt is almost as bad as not enough, as plug fouling, poor fuel economy, and sluggish throttle response are usually the results. It should be remembered that regardless of application, the pump circuit should provide just enough fuel to prevent stumble. Once this point is reached, the delivery of additional fuel degrades overall performance, particularly while the engine recovers from an over-rich condition.
Change the Nozzle, Alter the Rate
Circuit timing, however, rather than fuel volume, is frequently the culprit that’s causing a stumble. With a given volume of fuel to be discharged (the fuel contained within the pump chamber) and a piston—driven by a spring—that discharges through a fixed nozzle orifice, it is nozzle size that affects the fuel discharge rate (how fast the fuel is pumped into the engine). Squiring all the gasoline in the float bowl still won’t overcome a stumble if it is all consumed before the main system starts functioning. The idea here is to size the nozzle so that the accelerator pump continues to squirt fuel throughout the entire period during which the main system transitions from non to full-function. As nozzle diameter is decreased, the discharge duration is lengthened, and vice versa. Edelbrock offers an Accelerator Pump Nozzle assortment (part no. 1475), which contains a variety of sizes. If availability is a problem, the nozzles can be drilled with few of the ill effects that accompany main jet drilling. Nozzle size has no appreciable effect on total discharge volume, only the amount of time required to discharge that volume.
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When adjusting discharge duration, it is advisable to have several “squirters” on hand. Carter used to offer a nozzle package under two part numbers: 48P-379U applies to AFB and AVS models and 48P-38OU applies to Thermo-Quads. Three nozzles (also called accelerator pump clusters) are contained in each kit. These kits have obviously been out of production for quite some time, so these part numbers are useful only if you somehow happen to come across the parts to which they correspond.
Secondary Main Metering Systems
Except for the Thermo-Quad, all Carter and Edelbrock four-barrels use the same type of main jets on both primary and secondary circuits. Although there is usually a considerable size difference, owing to the use of metering rods in the primaries, the jets are interchangeable. Secondary rejetting is simply a matter of swapping the existing jets for others of different size.
Secondary jetting on the Carter Thermo-Quad is essentially the same as found in the AFB and AVS, except that primary and secondary jets are not interchangeable. The secondary “jets” are actually quite different in appearance, being “long hunks of brass,” rather than the more customary short, threaded insert. The unique secondary jet configuration owes in part to the Thermo-Quad’s phenolic main body, and the minimization of passages within it. Since fuel must be brought from the float bowl floor up to the discharge nozzle, a path must be provided—Carter used the jet to complete the circuit. Rather than screwing into the float bowl floor, the jet threads into the aluminum upper casting (the body that also houses the choke) and extends down to the bowl floor. It might best be thought of as a metering orifice at the end of a straw. Aside from its unusual appearance and the fact that it is held in position with an O-ring, it is identical in function to a standard screw-in jet.
Numbering System: There’s Logic Somewhere
Carter utilizes several part-numbering systems for jets, all of which contain information relating to orifice size, but they use a unique encoding. All WCFBs, most AFBs and AVSs, and 1974 and later BBD carbs use jets with a three-digit suffix that describes orifice size. (All Carter jet part numbers begin with “120-”.) If the suffix begins with a “3,” jet size is less than 0.100-inch; a “4” in the first suffix position indicates an orifice of 0.100-inch or greater. Thus a 0.098-inch jet would carry part number 120-398, while a 0.101-inch jet would be labeled 120-401. If you’re still with us, now we’ll get tricky. When referencing the jets used in AVS and AFB carburetors with three-step metering rods, a “5” in the first suffix position pertains to sizes over 0.100 inch and a “4” describes jets with diameters of 0.099 inch or less.
Thermo-Quad jets have a numbering system all their own, utilizing a four-digit suffix with either a “4” or “5” in the first position. The “4” relates to primary jets while a “5” indicates a secondary jet. Diameter is encoded in the next three digits, thus part number 120-4107 is a 0.107-inch primary jet. This numbering system applies to all Thermo-Quads except the 4000 series (which includes the Competition Series). These carburetors have two- rather than three-step metering rods, and a jet part-numbering scheme that defies comprehension. If you intend to work with this carburetor, only the metering rods should be changed. Part of the problem arises from the fact that at least two of the Competition Series Thermo-Quads were equipped with pressed-in rather than screw-in jets. These are marked with a three-digit suffix, making the part numbers 120-331 and 120-334. With pressed-in jets, all calibration changes must be made through a swap of metering rods.
Edelbrock’s jet part numbering scheme is thankfully much simpler. Jet sizes range from 0.077 to 0.119 inch and part numbers contain four digits (which applies to a pair of jets) and run sequentially from 1420 to 1435. Across the entire range, jet size steps up 0.003 inch per part number, except for jet numbers 1427, 1428, and 1429, which have diameters of 0.098 inch, 0.100 inch, and 0.101 inch, respectively.
Except for a few limited production versions of certain Carter models, all Carter and Edelbrock (Carterbrock?) four-barrel carburetors utilize either a counterweighted auxiliary throttle valve or a spring-loaded air valve to control airflow through the secondary bores. Conceptually, both arrangements are identical and function in conjunction with a mechanically actuated secondary throttle to provide smooth, stumble-free transitions to wide-open throttle. Since the auxiliary throttle valves used in WCFBs and AFBs do not completely seal against the bore, there is always some airflow initiated just as soon as the mechanically operated throttle valves are opened. As airflow increases, the counterweights are overcome and flow increases further.
With the auxiliary throttle valve counterweights controlling the actuation of the secondaries, in order to speed the secondary opening, some material must be removed from the weights. This is a simple modification as the weight is externally accessible on WCFBs, and may be easily removed from AFBs. Only small amounts of weight should be removed—a counterweight that is too light allows the air valve to flop open prematurely, causing the engine to stumble or hesitate.
In most cases, the counterweight is properly calibrated so that no modification is required. However, the larger the engine displacement in relation to the carburetor airflow, the more likely that throttle response will benefit from lightening the counterweights. And in some instances, especially where a low airflow carburetor is used on a large engine, it is possible to entirely remove the auxiliary throttle valve. However, before depositing such a valuable piece of carburetor memorabilia in the trash can, a road test is in order to ensure that the engine doesn’t stumble when the carburetor is brought to wide-open throttle, with the transmission in high gear (a condition of maximum load, most likely to cause a stumble).
Air Valve Adjustment
Adjustment of the secondary air valve used in the AVS and Thermo-Quad carburetors is much easier than modifying the counterweights used on auxiliary air valves. Since spring pressure holds the air valve closed, altering that pressure increases or decreases the opening rate. Both the AVS and the Thermo-Quad carburetors have a locking screw that must be loosened before the spring-adjusting screw can be turned. Whenever making an adjustment, the spring screw should be held while the lock screw is being loosened to prevent the spring from unwinding. If the spring does unwind, a new reference must be established and subsequent adjustments made from that point.
Bear in mind that carburetor tuners are not without “human error.” Developing a repeatable reference point before a trial-and-error tuning session is begun can save many hours of lost effort caused by an unwound air valve spring. Many racers form an initial reference point by unwinding the spring until the air valve rotates into a vertical position, then tighten the spring a particular number of “turns” from that point. In this case, if the spring accidentally unwinds, it can be returned to approximately the same position.
Two screwdrivers are the only tools required to adjust an AVS air valve spring, and while the same arrangement works on a Thermo-Quad, life is much easier when a special tool (once offered by Carter as part number 109P-397) is used. The Thermo-Quad spring adjustment screw is located within, rather than next to, the lock screw. Although a narrow-bladed screwdriver can be used to hold and/or turn the lock screw (once it has been broken loose with a wide-bladed screwdriver) while another screwdriver is used to hold the spring screw, three fewer hands are required when the Carter tool is used.
For applications where performance is of primary interest, spring tension should be adjusted to provide the quickest possible stumble-free opening rate. A considerably greater amount of tension is required to provide smooth transition from the primaries to the secondaries if the vehicle is a heavyweight. This is because the engine loads encountered may be sufficient to require full opening of the primary bores—and therefore partial opening of the secondaries—when climbing a hill or accelerating a bit more rapidly than normal. The resulting airflow in the secondaries may open the air valve when not desired (and produce the well-known “bog”) if spring tension is too tight.
Fuel Inlet System
Without an adequate fuel supply, the best carburetor in the world is reduced to a metering nightmare. It’s much like an athlete who doesn’t eat the proper food in adequate amounts—he simply can’t perform at 100% efficiency. Likewise, carburetor fuel metering circuits operate at less than optimal levels when there is insufficient fuel to go around. As a solution to this problem, many well-meaning carb tuners install the largest needle-and-seat assembly available, eliminate every filter and baffle in the system, and proceed to scratch their collective heads when the situation progresses from bad to worse.
In general, very little in the way of modification needs to be done to the fuel inlet system of any Carter or Edelbrock carburetor. As a minimum, larger needle-and-seat assemblies may be installed to ensure that there is no bottleneck as fuel enters the carburetor. Many AFB, AVS, and Thermo-Quad models are fitted with 0.0935-inch or 0.1015-inch assemblies at the factory. These sizes are far from restrictive for most applications, but larger diameter pieces are available. However, with this size of inlet seat, insufficient fuel delivery is almost always the result of problems located elsewhere in the system. With adequate fuel pressure, only a float level that is set too low, clogged inlet filters, or dirt lodged in the inlet seat can cause the carburetor to “starve.” More commonly, the trouble lies in a crimped line, sediment infested gas tank, or an inadequate fuel pump.
For typical high-performance use, installation of the optional 0.110-inch needle-and-seat assembly (Edelbrock part no. 1466) and setting float levels as recommended should provide trouble-free operation. When an AFB, AVS, or Thermo-Quad is used for race applications, a larger 0.12-inch or 0.125-inch inlet valve assembly may prove advantageous. These provide fuel flow of up to 110 gallons per hour (assuming there is a sufficiently capable fuel pump to back them up).
In addition to float level, float drop must also be checked. Generally, a small tang on the float determines the maximum distance that float may drop (lower into the float bowl) and this determines the distance that the needle is pulled away from the seat, affecting the fuel flow through the needle and seat assembly. If this tang were misadjusted, it would be analogous to an intake valve being only partially opened; flow is understandably reduced. Float drop may be easily checked by simply holding the air-horn assembly and first visually inspecting needle position with the float tang resting against the inlet seat. In this configuration, “float drop” is measured distance from the float to the air-horn assembly. In most situations, too much float drop rarely causes problems; to adjust, simply bend the tang to allow the float to drop to a lesser or greater degree, as required.
It is a basic fact that Carterbrock carburetors are not easily modified for all-out competition. As a result, comparatively few companies offer extensively reworked AFB, AVS, or Thermo Quad four-barrels or modification services. This situation might lead you to believe that nothing can be done to coerce a race engine equipped with one of these carbs into producing more power. Fortunately, that’s not true. Racecar performance can be improved by a few carburetor alterations that either increase airflow or improve fuel-handling capabilities.
Of all the four-barrels in question, the AFB lends itself most readily to major modifications. The AVS is a close second, but the Thermo-Quad is a distant third as its phenolic main body and double primary booster venturi make machining difficult or impractical.
A Little Here, A Little There
A slight airflow improvement may be achieved by altering the booster venturis, which are fitted to the primary throttle bores of AVS and all throttle bores of AFB carbs. In addition to “cleaning up” the outside diameter of the booster itself, it is also possible to narrow its support leg, which protrudes into the air stream. This work is best done with a small high-speed grinder. These tools remove material quickly yet are easily controlled to prevent cutting too deeply. A word of caution: The boosters are aluminum castings that have brass tubes running through their centers. When the main metering circuit is activated, the air/fuel emulsion flows through these tubes, the ends of which serve as discharge nozzles. If too much aluminum is removed from the booster support leg, it is possible to grind right into and through the brass tube, thereby turning a perfectly good booster into a useless fishing weight.
Narrowing the boosters isn’t the only means by which to increase airflow—reducing the height of the support leg also helps as it reduces the friction to be overcome by the air stream. Again, caution is in order, as grinding too far damages the brass fuel tube. While working on the leg, be careful not to grind the bottom of the booster venturi itself as the fuel-metering signal may be adversely affected. Once the leg has been properly shaped (a modified booster is illustrated in the accompanying photos), blend it into the outside diameter of the venturi with a smooth radius and carefully remove the casting lines. When all grinding has been completed, smooth the altered surfaces with fine-grit sandpaper. It should be noted that all this work will result in only a minor increase in airflow, so don’t expect a miraculous improvement in performance.
A Little More Here, A Little More There
Removing the choke and its attendant linkages enhances airflow even further. On the AFB, AVS, and Thermo-Quad, choke plate position is controlled by a link rod above the secondary venturis. Therefore, when the choke is eliminated, these obstructions are removed from both the primary and secondary barrels.
Choke plates are held in their shafts by two brass screws that are staked (flattened with a punch) on the threaded end to prevent them from vibrating loose. When removing these screws, increased resistance will be encountered as the staked portion passes through the threads in the shaft. It isn’t necessary to grind the staked threads; continued rotation in the counter-clockwise direction will usually “reform” the screw end as it is extracted from the choke shaft. But this only works properly if the screwdriver tightly fits the screw slot. Again, the resultant airflow improvements will not be dramatic, but every little bit helps.
Don’t be too hasty to deposit the choke pieces in a trash bin—you may want to reinstall them at a later date. If you do, restake the choke plate screws—after they have been cinched down—by repeening or “squashing” them with a pair of vise grips. If you use a hammer and punch, be sure to place some form of support under the choke shaft so it isn’t bent or damaged in the operation.
More Fuel for the Fire
A race engine demands fuel like a wino demands distilled spirits. Consequently, the fuel delivery system must be reworked to ensure that it is not restrictive. As such, a high-capacity fuel pump is a necessity. A number of manufacturers offer electric and mechanical fuel pumps that can deliver more than enough fuel for any engine producing less than 1,000 horsepower, so ensuring an adequate supply is not the challenge it once was. In most instances, a good deal of pressure comes along with increased volume, so use of an appropriate pressure regulator may be necessary to prevent the inlet needles from being literally blown off their seats. Typically, pressure settings between 6 and 7 psi prove to be ideal.
With a minimum of 3/8-inch (-6 AN) fuel lines connecting the gas tank, fuel pump, and carburetor, the next step toward ensuring adequate fuel during “pedal to the metal exercises” is reworking the carburetor inlet circuit for maximum capacity. As previously noted, most Carter and Edelbrock carbs incorporate needle-and-seat assemblies having a 0.0935-inch diameter orifice. (Carter part numbers are 25-1086 for Thermo-Quads and 25-860 for the others.) For competition work, Carter used to offer needle-and-seat assemblies with a 0.120-inch, orifice for the WCFB, AFB, and AVS (part numbers 25-862), or a 0.125-inch opening for Thermo-Quads (part number 25P-1091). The largest needle-and-seat assembly offered by Edelbrock has a 0.110-inch inlet (part number 1466). Keep in mind that even in race-only applications, the biggest isn’t necessarily the best. Larger needles may have difficulty sealing with high inlet pressures; frequent readjustment of the float level may be required as a higher fuel level may develop in the bowl before the needle is forced against its seat with sufficient pressure to stop fuel flow.
The fuel demands of a fire-breathing race engine may be better satisfied by a dual-feed fuel inlet conversion; a modification readily performed to AFB and AVS carburetors. The air horn casting of these carburetors is designed to allow fuel to be admitted from either the left or right side. In original configuration, only a single fitting is used, so bringing dual-feed happiness to an AFB or AVS is simply a matter of drilling and tapping the unmachined inlet boss. However, with some Edelbrock carburetors (which have both inlet openings machined), the conversion is as simple as removing a plug and installing an inlet fitting in its place. The fuel passage itself measures approximately 3/8-inch in diameter. Once this hole is drilled through, the inside diameter of the boss may be threaded with a 9/16-18 or 5/8-20 tap. This allows installation of either a standard fuel line adapter or the appropriate AN fitting if braided line is used. AFB and AVS carbs produced by Edelbrock use 5/8-20 inlet fittings and some have a plug pressed into the unused inlet opening. This plug has to be removed prior to machining, which in all cases should be performed by a competent machinist with the appropriate equipment (i.e., mill, drill press, etc.)
Developing “optimum performance” is possible only through a series of trial-and-error tests. Carburetor tuning is greatly facilitated by having a selection of the necessary jets, metering rods and springs on hand. You may also want to have some needle-and-seat assemblies and accelerator pump discharge nozzles on hand.
Drag strip tuning frequently requires extensive experimentation with accelerator pump discharge nozzle diameter. In some instances, drilling of the nozzles is required to obtain the correct flow requirements. Remember that alteration of orifice size does not affect the volume of fuel discharged through the accelerator pump nozzle, only the duration of flow; as orifice size is increased, the same amount of fuel is squirted in a shorter time. While it might seem that larger is unequivocally better, it is possible to impair performance by admitting fuel too quickly. If nozzle diameter is too large, a cloud of black smoke appears as the car leaves the starting line. This may very well be followed by a stumble as the engine encounters a momentary lean condition, occurring when the pump shot is dissipated before adequate fuel emulsion begins to flow through the main metering circuit. In order to avoid such a situation, start with the smallest discharge nozzle orifice and work towards the larger sizes. Optimum is obtained by using the smallest-diameter squirter that allows the car to cleanly leave the starting line without stumbling or hesitating. Once beyond this point, a quicker discharge rate is counter-productive, as fuel mixture will momentarily become excessively rich.
A certain amount of time is required to get any fluid moving. Consequently, activation of the power-enrichment circuit must be timed to avoid a transient lean condition as the engine is brought from part-throttle/no-load to full-throttle/full-load operation. This is where the metering-rod-piston springs come into play. The stiffness of these springs determines the manifold vacuum level at which the metering rods are raised to the power step. And in a race engine, it is important to adjust spring pressure so that enrichment is not engaged during idle, a common problem encountered with the low idle vacuum caused by racing camshaft profiles.
If you’re using Carter springs, you need to test them to establish the manifold vacuum levels at which they operate. This is most easily accomplished by using a rubber hose or tapered fitting that is connected to a hand-held vacuum pump on one end and the channel through which manifold vacuum reaches the power piston housing on the other. Then remove the metering rod chamber cover, work the pump, and note the vacuum reading at which the metering rod begins to move up or down. This is the point at which the economy-topower transitions occur.
Once this is determined, connect a vacuum gauge to the engine and check manifold vacuum at idle. In order to avoid an overly rich condition, the idle vacuum should be greater than the vacuum required to hold the pistons in their down position. By way of example—if an engine idles at 12 in/Hg, the metering rod springs should “spring into action” at 10 to 10-1/5 in/Hg. (Keep in mind that many gauges aren’t particularly accurate.)
With optimized fuel circuit calibrations, it is entirely possible that a faster secondary air-valve opening-rate may be tolerated. As mentioned earlier, the spring-loaded valve used on AVS and Thermo-Quad carbs is easily adjusted. The AFB presents a different situation in that opening rate is altered by removing material from the counterweight. When introducing the AFB air valve weights to a grinder, proceed judiciously—it’s much easier to remove than to replace weight material.
Written by Dave Emanuel and republished with permission of CarTech Inc
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