Holley Carburetors. Mike Mavrigian

       Removing or installing the needle and seat assembly must be done with care to avoid damaging the assembly’s O-ring.

      Some carbs have a brass screw-in plug that must be removed. Fuel level is adjusted until the fuel is at the bottom of the sight hole. Other models have a glass sight window, which allows you to observe the fuel level without having to remove a plug. On 4160s with side-hung floats, the fuel bowl must be removed and turned upside-down; adjust the float so that the top of the float surface is parallel to the roof of the bowl.

       This is a front view of a metering block equipped with main jets. The slot on each jet allows servicing with a flat-blade screwdriver, although a dedicated Holley jet driver is preferred because it avoids accidental slot deformation. See Chapter 4 for information about specialty carburetor tools.

       Main jets are available in a wide range of orifice sizes. All jets share the same exterior shape, size, and thread. A selection of jet sizes is shown here, threaded into a billet aluminum jet card from AED. A jet organizer such as this allows a neat and orderly method of storing extra jets if you plan extensive tuning.

      Each fuel bowl is vented internally to the air horn by a vertical vent tube in the main body, which allows the release of excess fuel vapor.

Jets

      Screw-in metering jets are exactly what the name suggests. They meter the amount of fuel that flows from the fuel bowl into the main fuel circuit; larger metering holes allow more fuel to flow and smaller jets allow less fuel to flow. Fuel runs through the jets into the metering block and then upward along the channels to where it meets with air from the air bleeds. At this point, the emulsification process begins and the fuel mixture enters the main body. The pressure drop in the booster venturi pulls air from the air bleeds and pulls fuel from the jets.

       Main jets are machined from solid brass. The metering orifice sizes range from .040 (jet number 40) through .128 inch (jet number 100). Selection in jet sizes is mostly in .001-inch increments.

       The main jets have been removed from the metering block, but the power valve is in place. The main jet thread’s size is identical for all jets, at 1/4-32. Never add any type of thread-locking compound to jet threads. Jets are tightened snugly to 30 to 40 in-lbs.

       The jet number is stamped into the side of each jet. This jet is a number 71, which indicates that the metering orifice is .076 inch.

Idle Circuit

      The idle circuit supplies the air/fuel mixture for engine operation at idle and low engine speed. The purpose of a secondary idle circuit is to maintain a constant fuel level in the secondary fuel bowl. Both primary barrels use identical idle circuits and both secondary barrels use identical idle circuits. However, the primary and secondary idle circuits do differ from each other.

       Primary Idle

      Fuel flows from the primary fuel bowl through the main jets, into a small, horizontal idle-feed passage that leads to a vertical idle-well passage. It then flows past an idle feed restriction, through another horizontal passage, and is eventually mixed with incoming air from the idle air bleed. The fuel then flows down a vertical passage to the bottom of the main body, where it splits in two directions; one path goes to the idle discharge passage and the other goes to the idle transfer passage and constant-feed port. The mixture that flows to the idle-discharge passage flows past the tip of the idle mixture adjustment needle screw, then through the main body and to the throttle body, where the mixture is discharged into the throttle bore, below the closed throttle plate.

      The mixture flows unrestricted in the passages leading to the idle-transfer passage and constant-feed port, through which it exits. When the throttle plate is closed, no fuel is discharged through the idle transfer slot. The transfer slot acts as an air bleed directly above the idle constant-feed port and serves to further lean out the air/fuel mixture.

      As the throttle plate opens and engine speed increases, the idle-transfer slot is exposed to intake manifold vacuum and fuel is discharged from the transfer slot. As the throttle plate continues to open, engine speed and airflow through the carburetor increases; it is increased even further by the venturi effect. As airflow increases, the main metering system begins to operate and the idle system begins to taper off; this provides a smooth transition from idle to engine operating speeds.

       The vertical idle air bleed in the metering block intersects the short horizontal passage that locates the idle-mixture screw.

       Secondary Idle

      All Holley 4-barrel carburetors have an idle circuit built into both the secondary and the primary sides. Some secondary idle circuits are predetermined by design and are not adjustable, but some 4150 carbs with mechanical secondaries have the same idle adjustments that are found in the primary side. Some (but not all) carb models in the 4150 series have idle-mixture screws in the secondary, as well as in the primary, metering blocks. This feature is provided on an individual basis according to the part number.

      An idle circuit in the secondary side controls fuel flow through the secondary bowl; it helps maintain proper fuel level, even if the secondary throttle plates are not open. To set up a carb with both primary and secondary idle-mixture screws, gently screw them in until they seat. Then, back them out one full turn. When the engine is running, adjust the idle speed. Depending on the design of the intake manifold, it is easy to tune for a variety of specific applications with this four-corner mixture adjustment setup.

Main Metering

      When the throttle is partially open, the main metering system on the primary side meters the fuel flow from the fuel bowl through the main jets and into the main fuel well. Fuel flows past the main-well air bleeds, where it mixes with air. This air/fuel mixture then exits through a horizontal passage to the discharge nozzle in the booster venturi, where it mixes with incoming air. Because this air/fuel mixture is lighter than liquid fuel, it responds faster to changes in venturi vacuum and vaporizes more easily when discharged into the airstream. Throttle plate position regulates the amount of the air/fuel mixture entering the intake manifold, and as a result, it regulates engine speed.

       This comparison of a 4160 carb (left) and a 4150 carb (right) clearly shows the secondary metering plate on the 4160 and the secondary metering block on the 4150.

      On the secondary side, the main metering system operates in a similar manner. On 4160 carbs, fixed (or predetermined) restrictions are machined into the metering plate. Idle fuel wells branch off each main well. Fuel for the idle and idle transfer system enters the main well through the plate restrictions and then it travels through the idle well and the idle restriction. Finally, it is mixed with air entering from the secondary air bleeds.