Holley 4-barrel carburetors are dual-stage, downdraft units. A dualstage carb has a primary side that supplies the air/fuel mixture throughout the entire stage of engine operation, and a secondary side that operates only when the engine demands a greater quantity of air and fuel.
This Tech Tip is From the Full Book, HOLLEY CARBURETORS: HOW TO REBUILD. For a comprehensive guide on this entire subject you can visit this link:
SHARE THIS ARTICLE: Please feel free to share this article on Facebook, in Forums, or with any Clubs you participate in. You can copy and paste this link to share: www.musclecardiy.com/tech-tips/holley-carburetor-operation-for-rebuilds/
The basic components include a main body, baseplate (or “throttle body”), primary and secondary fuel bowls fitted with adjustable floats, accelerator pump and accelerator pump nozzle (or “squirter”) on the primary bowl, primary and booster venturi with primary discharge nozzle, throttle plates, idle fuel passage, and a primary metering block with screw-in jets on the primary side. The 4150 series has a secondary metering block; the 4160 has a secondary metering plate. The 4150s with mechanical secondary operation have an accelerator pump and squirter on the secondary bowl.
The venturi (or “barrel”) passage has a double-taper design: The larger area at the top narrows and then enlarges above the throttle plate. This shape creates higher velocity and lower pressure at the narrow section; the vacuum effect draws in fuel and air.
The fuel mixture is pre-atomized, mixing with air from the small air-bleed orifices in the top of the main body as it passes through the discharge nozzle in the venturi booster. It is emulsified further as it passes through the high-speed column of air, resulting in the air/fuel mix to change the fuel stream into tiny droplets in a “mist” configuration. The air bleeds are sized to work in conjunction with the vacuum effect in the venturi (called the venturi effect), which increases as the throttle is opened and is greatest at wide-open throttle (WOT). The venturi and booster venturi work together; the booster venturi aids in generating additional pressure drop at reduced air speed and increasing the fuel flow.
Several booster designs have been used in Holley carburetors. They provide more or less restriction; signal strength increases as restriction increases, and this affects the pressure drop. In addition, fuel distribution and degree of atomization vary according to the signal.
A “standard” booster used today has a straight leg and a single small orifice passage for fuel entry into the booster’s cavity. A “down-leg” booster includes an angled leg, which provides less restriction.
Annular boosters, found in some 4150 series and all 4500 Dominator carbs, have a series of small orifices around the inside of the booster. They create a stronger signal that is especially useful with engines that have long-duration camshafts and low manifold vacuum at low engine speeds.
Holley carbs come in many designs. The 4160s have a fuel inlet at the primary bowl; a transfer tube then supplies fuel to the secondary bowl. The 4150s include dual fuel inlets, with an inlet at each bowl. Fuel travels through the needle and seat assembly and the bowl fills.
Regardless of series, as the fuel level inside the bowl rises, the float rises. Pressure from the float causes the needle to seat, cutting off the fuel flow. As the engine uses the fuel, the fuel level drops, lowering the float. Pressure from the dropping float unseats the needle, allowing fuel to flow into the bowl once again. The float level must be adjusted correctly so that the basic fuel metering system operates as designed.
On carburetors equipped with center-hung floats, float adjustment is handled externally via needle height adjustment. To make an adjustment, loosen the slotted plug (which is the lock), turn the hex to make the adjustment, and then tighten the lock. A sight port on the side of the fuel bowl allows you to monitor fuel height while the engine idles.
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.
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.
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.
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.
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.
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.
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.
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.
During hard acceleration, the increase in airflow results in leaning the air/fuel mixture. A power valve is located in the primary metering block to provide the additional fuel to counter this and to enrich the mixture. The power valve has an internal diaphragm and is operated by vacuum supplied through passages in the throttle body baseplate and the main body. At idle or under load, there should be enough vacuum to keep the power valve closed.
As manifold vacuum drops under high-speed conditions, the spring in the power valve forces the diaphragm to open, allowing extra fuel to flow through the power valve. The fuel then travels through the restrictions in the metering block and into the main well, joining the main fuel flow, which richens the mixture. After engine speed is reduced, the manifold vacuum rises and causes the power-valve diaphragm to overcome its spring pressure and close.
The fuel bowl is vented internally to the air horn by a vent tube in the carburetor body, which allows the escape of excess fuel vapors. With the primary fuel bowl removed, you can see a small rectangular passage at the top center face of the metering block. This opening aligns with a small rectangular passage in the main body that enters the vertical vent tube. If the float is set too high, or if the needle-and-seat assembly are stuck (or if a small crack is in the metering block or main body), fuel can spurt out of the vent tubes and send excess fuel into the throttle bores, causing a bog.
Under hard acceleration or when driving in off-road conditions where extreme vehicle angles are encountered, fuel from the primary bowl can suddenly be crammed into the vent passage and out of the vent tube. A vent “whistle” is standard on some Holley carbs, or one can be added easily to an existing carb. This vent whistle extends into the fuel bowl area. If, under hard acceleration, a shock of fuel is slammed into the passage, the excess fuel is diverted through the vent whistle and back into the fuel bowl.
Another method of avoiding excess fuel spill through the main vent tubes is with a radiused U-shaped crossover vent tube that connects to both the primary and secondary main vent tubes. This tube has small holes to allow air venting, but it provides a longer travel path if excess fuel tries to slosh out of the bowl through the main vents. This type of crossover vent tube is popular among off-roaders and for some marine applications where severe operating angles are commonly encountered.
Vertical vent tubes are located at the front and rear of the main body. Be aware of clearance issues between the top of the vent tubes and the air cleaner lid. Opinions vary regarding an acceptable clearance, but generally speaking, you should have at least 3/8 inch between the top of the vent tubes and the air cleaner lid. This may be a concern when dealing with a drop-base-style air cleaner where the lid is closer to the air horn. Some vent tubes have an angled cut at the top to minimize the risk of blocking the vent tubes. Other carbs (usually those with no choke) include shorter vent tubes that are straight-cut so they’re short enough to stay well away from the air cleaner lid.
In addition, the primary and secondary sides of the main body each have four small holes at the top that serve as air bleeds. The two inboard holes bleed the main circuit; the two outboard holes are for the idle circuit. These air bleeds must remain clean and free of debris.
An accelerator pump is a small lever-operated diaphragm pump located on the underside of the fuel bowl that is connected to the throttle linkage. All Holley 4-barrel carburetors have a primary-side accelerator pump located on the driver’s side of the primary fuel bowl. The 4150 carbs with mechanical secondary operation, and all 4500 series carbs, have an additional accelerator pump on the passenger side of the secondary fuel bowl, which is why they are called Double Pumpers.
The accelerator pump provides a quick, brief shot of fuel into the venturi as soon as the throttle is moved. Fuel is sent via the pump action through the accelerator nozzle (squirter) that is located immediately above the venturis. The squirter nozzle has twin orifices, with one aimed at each barrel on the primary and/or secondary barrels.
During the initial hit on the throttle, engine vacuum isn’t high enough to draw fuel through the venturis quickly enough, which results in a momentary stumble. The quick shot of fuel from the accelerator pump compensates and provides a smooth transition from initial throttle to wider throttle opening.
The pump’s operation can be tuned by changing the pump itself, the pump’s squirter nozzle, and/ or the cam that’s mounted on the throttle lever. The cam determines the amount of “lift” that takes place at the accelerator pump arm, similar to an engine’s effective valve lift (the ratio of the rocker arm increases the valve lift above the camshaft lobe lift). A selection of profile plastic cams is available to tune the degrees of lift at the accelerator pump arm.
Variables, including vehicle drive gearing, vehicle weight, camshaft timing, carburetor size, engine torque, etc., influence the length of time involved in the transition period. As engine load is introduced, both airflow and engine vacuum are low, and the need for the shot of fuel from the accelerator pump increases. As the lag time increases, a longer shot of fuel is needed to maintain smooth transition as the throttle is opened.
Depending on the specific carburetor, it has either a single or dual fuel inlet. The 4160 carbs typically have a single fuel inlet; fuel from the fuel pump enters only the primary fuel bowl and is then transferred to the rear bowl via an external fuel transfer tube. The 4150 carbs, whether they are Double Pumpers or not, have dual fuel inlets; a feed port is at each fuel bowl.
A single-feed inlet typically includes a banjo fitting that positions a barbed slip-on fuel hose leg at 90 degrees to the side of the fuel bowl. The banjo fitting has a sealing crush washer at each side of the round banjo end. One washer is between the banjo and the fuel bowl and another washer is between the banjo head and the bolt that secures the fitting to the fuel bowl. The banjo bolt runs through the round fitting head and has a fuel passage in the side of its shank. It also has an exit passage at the shank tip, allowing fuel to flow into the banjo fitting, turn 90 degrees, and then enter the fuel bowl.
Dual-feed carburetors that have a fuel inlet for each fuel bowl are found on carbs equipped with center-hung floats. A fitting with male threads engages the female-threaded hole in the fuel bowl; it also has a female-threaded entry hole that accommodates a line/fuel log fitting.
In a 4160 fuel bowl the female thread size that accepts the banjo fitting is 9/16-24. The fuel inlet fitting thread size in each center-hung fuel bowl is 7/8-20. Depending on how you wish to plumb the fuel line to the bowls, you can either select a fuel log with ends that thread into the adapter fittings or that have 7/8-20 ends that thread directly into the fuel bowls without the need for the original fittings.
A Holley banjo fuel inlet bolt requires an 11/16-inch wrench. The majority of 4150 and 4500 carb fuel bowl inlet fittings require a 1-inch wrench, but some carbs, such as the Ultra series, require a 3/4-inch wrench (same bowl thread size, but a slightly smaller and easier-to-access hex). The 1-inch hex fittings provide less-than-ideal access for a wrench because the hex is very close to the fuel bowl outer wall. The smaller hex allows much easier access.
Mechanical versus Vacuum Secondary Operation
To avoid a stumble when the secondary plates open, the secondary plates must open at a slightly slower rate than the primary plates. This is true with either mechanical or vacuum operation.
When the primary throttle plates open to approximately 45 degrees, the secondary plates begin to open. The secondary opening rate must be quicker than the primary rate so that both reach WOT at the same time; it provides a smooth transition as engine speed and fuel demand increases. This transfer from primary to secondary throttle plate movement is called progressive movement. If the pedal is quickly and aggressively stepped on and the secondaries open before enough fuel is pulled through the venturis, the secondary fuel bowl’s accelerator pump provides a quick shot of fuel to ease the transition, avoiding a stumble.
Vacuum secondary operation, which is much more suitable to a street-driven application, controls the secondary plate opening via a diaphragm-equipped vacuum unit mounted on the passenger’s side of the carburetor. The vacuum system opens the secondary throttle plates according to engine demand and is not dictated by the driver’s right foot (that is, not mechanically). A spring inside the vacuum unit, on top of the diaphragm, keeps the diaphragm closed.
As the airflow through the primaries reaches a certain point and as engine speed and venturi vacuum increases, a vacuum signal from the primary venturi is routed to the vacuum unit, pulling the diaphragm open and overcoming the spring. When additional airflow begins to build in the secondary side, additional vacuum pull forces the diaphragm to open farther, opening the secondary plates farther.
Manual and Electric Choke
Choke operation is located on the passenger’s side of the carburetor. If the carb is equipped with a manual choke, a control cable is required to allow manual choke plate position control from the driver’s area. An electric choke automatically controls the position of the choke plate when the engine is started. The temperature-variable flat-wound spring inside the choke housing contracts and relaxes based on temperature, controlling the choke plate position. HP-series carbs (including the 4500 Dominator series) have no choke provision, and are designed for optimum performance. The elimination of a choke horn and choke plate increases airflow to the venturis.
A choke system is needed for street operation for which the driver expects the engine to start quickly and attain an acceptable idle when the engine is cold. When the metal surfaces inside the carburetor are cold, fuel doesn’t atomize readily. Also, the starting attempt occurs when there is little vacuum signal, which means that there is little vacuum draw to pull fuel out of the bowl. A choke plate positioned above the primary venturis is closed to increase the vacuum signal, which helps to draw fuel out of the idle and main circuits. With less air and more fuel, a slightly rich mixture is created that helps the engine to start and run until the metal surfaces and the intake manifold reach an acceptable rise in temperature, at which time the choke plate is opened.
In warm ambient temperatures, a choke may not be needed at all. A few pumps of the throttle pedal causes the accelerator pump to send fuel into the venturis, allowing the engine to start, although the engine may idle/run a bit rough until the operating temperature rises to the point where fuel is atomized.
With a manual choke, the driver has immediate control over the position of the choke plate. With an electric choke system, specifically the integrated choke system on Holley carbs, the choke plate is held in the closed position when the heating element inside the choke housing is cold. The choke housing is connected to a 12-volt ignition circuit source.
Obtain the 12-volt power from the ignition circuit only. Do not take 12-volt power from the coil. When the engine is started (aided by the closed choke plate), the electrical heating element begins to warm. As the temperature inside the heating element increases, the choke plate begins to open and attains full-open position after the heating element reaches its designated temperature. A vacuum tie-in between the choke base housing provides air to prevent the choke’s heating element from overheating.
The electric choke is adjustable by loosening the three screws that secure the plastic choke housing to the choke base. The plastic choke-housing cap is rotated clockwise or counterclockwise to adjust choke operation (caps are marked “rich/lean” with arrows indicating which direction richens or leans).
4150 Vacuum and Mechanical Secondary Carbs
The 4150 has a dual-feed center-hung fuel bowl at both the primary and secondary sides. When equipped with vacuum secondary, no accelerator pump is located at the secondary side. A vacuum unit mounted to the passenger’s side controls the secondary throttle shaft. The baseplate (also called the throttle body) has a large vacuum port for positive crankcase ventilation (PCV) operation and a smaller vacuum port for accessory use.
Many 4150s also have a second large vacuum port at the rear for power brake booster operation. On the passenger’s side of the primary metering block is another small vacuum port for timed/spark advance. At idle, this port has little or no vacuum signal, which prevents the distributor from advancing prematurely. If your distributor has mechanical advance, this vacuum port must be plugged.
A curb-idle adjusting screw is located on the driver’s side. It allows you to set engine idle speed by opening and closing the primary throttle plate to control the amount of air entering the primary venturis.
On the passenger’s side, a fast-idle cam adjusting screw is also included on models with electric choke.
The main body has a choke horn and choke plate to control airflow during engine warmup.
Under the choke plate are the two primary venturis and boosters, as well as an accelerator pump discharge nozzle, or squirter. On models equipped with vacuum secondaries, there is no secondary accelerator pump or discharge nozzle. The primary and secondary sides each have four small holes at the top that serve as air bleeds. The two inboard air bleeds are for the main circuit and the two outboard holes are for the idle circuit. The primary metering block has two jets to control normal fuel flow and a power valve that acts as an auxiliary fuel supply during acceleration. Idle mixture is adjusted via a mixture adjustment screw on each side of the metering block.
All 4150 carbs have dual-feed center-hung fuel bowls. The floats are secured with center-mounted hinges. Each float presses on a needle and seat assembly. The float and needle/seat work together to control the fuel level in each bowl. On the top of each bowl is a nut that is used to adjust float level.
An accelerator pump arm is attached to the baseplate on the bottom driver’s side of the primary side. This arm presses against the accelerator pump’s diaphragm. The pump is activated every time the throttle is pressed; it sends fuel to the discharge nozzle immediately. A 4150 equipped with vacuum secondary operation does not have an accelerator pump on the secondary side. In fact, secondary accelerator pumps are only found on carbs with mechanical secondary operation. On a carb equipped with a vacuum-operated secondary, the vacuum unit handles control of opening and closing the secondary throttle plate. As the engine runs and venturi air velocity increases the vacuum signal in the venturi, the vacuum assembly automatically starts to open the secondary throttle plates when needed to supply the proper amount of fuel. To fine-tune secondary operation, you can change the vacuum assembly spring to a spring of a different rate.
A connecting link ties the rear throttle plates directly to the primary throttle lever. The 4150 Double Pumper with mechanical secondary operation allows you to manually control the opening and closing of the primary and secondary plates as you operate the throttle lever.
Written by Mike Mavrigian and Posted with Permission of CarTechBooks