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Carb Tuning
What do Chevrolet's dual-quad 409 and Chrysler's 426 Street Hemi have in common?
February, 2009
By Matthew King
Photography by Matthew King
 Left: This drawing illustrates...  Left: This drawing illustrates what occurs when manifold vacuum is high under low-load situations like idle, cruise, and light acceleration. The step-up piston and rod are pulled down against the resistance of the spring, and the thick part of the rod enters the orifice of the jet. This reduces the area that fuel can flow through and leans the mixture. Right: Under high loads such as heavy part-throttle or wide-open-throttle operation, manifold vacuum drops, and the piston "steps up" to the thinner power step of the metering rod. This is why the thick part of the rod is called the cruise step and the thin part is called the power step. |  |  The beauty of the Performer...  The beauty of the Performer carb's design is that metering rod changes can be made quickly without disassembling the carb. Loosen the hold-down plate screws, slide the cover out of the way, and the piston and rod will pop right out. |  You can tune the point when...  You can tune the point when enrichment takes place by changing step-up springs. This is generally done to cure mid-throttle drivability problems. Various color-coded springs are available, with vacuum ratings ranging from 3 to 8 in-Hg. The more vacuum it takes to activate the spring, the slower the enrichment comes in. Most Edelbrock carbs come with 5 in-Hg-rated springs in the base calibration. |  To remove the air horn for...  To remove the air horn for access to the jets and floats, the metering rods and step-up pistons and springs must be removed, followed by the eight screws that attach the air horn to the main casting. The air horn should always be set upside down so the floats and needle-and-seat assemblies don't get damaged. |  The primary and secondary...  The primary and secondary jets are located inside the main wells on either side of the main casting, and they can be removed with a large, well-fitting screwdriver. Jet changes are the only tuning step that requires any carb disassembly, and one nice feature of the Performer is that gas doesn't spill all over the place when you open the carb. |  Edelbrock jets are marked...  Edelbrock jets are marked with a three-digit part number that indicates the diameter of the metering orifice. Jet sizes starting with a 3 are smaller than 0.100 inch; jets beginning with a 4 are larger than 0.100 inch, so this 389 jet has an 0.089-inch orifice. |  Edelbrock has developed calibration...  Edelbrock has developed calibration charts for each of its carbs that map out the rod and jet changes required to go richer or leaner. The base calibration is at the center of the chart. Moving up and to the right richens the calibration; moving down and left leans the carb. To use the chart, decide how many stages you want to go, then cross-reference the number in the nearest circle to the chart to find out what jet and rod combo you need. These charts are printed in the owner's manual, available free from Edelbrock, for each carb along with a list of jet and rod part numbers. |  Metering rods are also marked...  Metering rods are also marked with a part number than indicates the diameter of each section of the rod. This 7547 rod has a cruise step with a 0.075-inch diameter and a power step with a 0.047-inch diameter. Edelbrock sells calibration kits for each carb model that contain an assortment of jets, rods, and step-up springs for tuning. |  Proper setting of the floats...  Proper setting of the floats is important because the level of gas in the bowls controls the supply of fuel to all the circuits. The float level is set by inverting the air horn and measuring the distance between the gasket surface (with the gasket in place) and the top of the brass float. This distance should be 7/16 inch. |  The float level is adjusted...  The float level is adjusted by carefully bending the float lever arm with a pair of needle-nose pliers. Always do this with the float removed from the carb so the needle-and-seat assembly isn't damaged. If the float level is too high, it can cause flooding or stalling on hard stops. Too low a float level may cause hesitation on hard acceleration if the primary jets get uncovered; dry-bowl starting problems may also occur. |  Float drop is the distance...  Float drop is the distance the float falls away from the needle and seat with no fuel in the bowl, and it's measured from the gasket surface to the top of the float with the air horn right-side up. The distance should be to 1-11/4 inches. |  The amount of float drop is...  The amount of float drop is adjusted by bending the thin tab at the end of the float lever arm. If the float drops too far, the needle may come far enough off the seat to bind during reassembly, resulting in flooding. If the float doesn't drop far enough, fuel flow into the carb is restricted. |  The Performer's secondary...  The Performer's secondary throttle blades are mechanically operated and don't start to open until the primary throttle blades have opened about 65 percent. Airflow through the secondaries is controlled by a weighted flapper-type air door called the auxiliary air valve, which prevents the carb from bogging down when the accelerator is floored at low speeds. As air velocity through the carb increases, the valve is pulled open, allowing the secondaries to operate fully. |  As the secondaries begin to...  As the secondaries begin to crack open but before the air door fully opens, fuel is drawn from the secondary initial discharge port located at the rear of each secondary bore venturi. This prevents a bog or stumble, similar to the way a Holley double-pumper provides an additional accelerator-pump shot to the secondary circuit, but it relies on vacuum signal rather than a pump. Once the air valve is fully open, fuel is metered through the high-speed discharge nozzle located in the booster venturi. |  The single primary accelerator...  The single primary accelerator pump provides an extra shot of gas when the throttle is depressed for quick acceleration. Moving the linkage rod to a higher hole lengthens the pump stroke and delivers more fuel. |  At idle, the air/fuel mixture...  At idle, the air/fuel mixture enters through the vertical transfer slot in the throttle bore. The volume of the mixture is determined by the large idle adjusting screws located at the front of the carb. |  The richness of the air/fuel...  The richness of the air/fuel mixture at idle is determined by the thin idle jet tube (right), shown with the primary venturi assembly upside down and removed from the main casting. If this tube gets clogged by dirt, the carb wont idle properly, so always use a quality fuel filter. The larger tube on the left supplies fuel from the primary jet to the high-speed circuit. |  The Performer on the left...  The Performer on the left is a model 1406 rated at 600 cfm; the carb on the right is a 1407 750-cfm unit. Both share the same size secondary bores, but the primary bore is smaller on the 600. The Performer is a square-flange design and is drilled for two baseplate bolt patterns. The larger pattern matches the Holley pattern and fits on any square-bore manifold. The smaller pattern was used for some OEM AFBs in the 60s. |
They were both factory-equipped with Carter AFB carburetors, which debuted back in 1957 and were also used on Avantis and many small-block Chevys prior to the introduction of the Rochester Quadrajet in the mid-'60s. When car crafters envision a performance carburetor, Holley's 4150 and 4160 series double-pumper and vacuum-secondary carbs often come immediately to mind. But long before Holleys dominated the street and strip, Carter's Aluminum Four-Barrel (AFB) carb was there, and it's never gone away. The AFB design still enjoys a loyal following, and for good reason. It's a simple carb to tune and rebuild, and it makes great power. Today, the basic design of the AFB has been adopted by Edelbrock as the basis of its Performer series of carburetors. Performers, like all AFBs, use metering rods attached to spring-loaded step-up pistons to provide fuel enrichment as the engine transitions from light-load, low-fuel-demand situations like cruising and light acceleration to high-load, high-fuel-demand situations like wide-open throttle acceleration. Metering rods can be swapped in minutes without disassembling the carb and allow you to richen or lean the mixture by up to about 8 percent in either direction without changing jets. Edelbrock's out-of-the-box calibrations will generally put you in the ballpark if you choose the right part number for your engine, but you still may wish to dial the carb in on your own combination to achieve its maximum potential. Follow along as we show you the basics of the Performer's operation and calibration.
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