Читать книгу Holley Carburetors - Mike Mavrigian - Страница 9
ОглавлениеHolley 4-barrel carburetors are dual-stage, downdraft units. A dual-stage 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.
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.
This Holley 4-barrel carb 4150 (PN 4779) is disassembled into its basic components. This carb has mechanical secondaries and a manual choke. At the bottom center is the baseplate, also called the throttle body. At the top center is the main body. The primary fuel bowl and metering block are at the right and the secondary fuel bowl and metering block are at the left.
This is a baseplate from a 4150 Double Pumper. The baseplate, also known as the throttle body, is the foundation for a carb and incorporates the throttle shafts and throttle plates.
Venturi boosters come in different designs. These are “down-leg” boosters; note the angle of the passage body. The inside of the booster air passage hole is a small fuel orifice.
The fuel hole in this straight-leg booster is located on the underside of the small passage pipe that runs through the center of the booster air passage.
This 4500 Dominator is equipped with annular boosters. The inside of the booster’s main air passage has a series of fuel orifices that runs around the inside diameter of the air passage.
Here, you can see some of the small fuel orifices inside the annular booster.
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.
This close-up of a side-hung float clearly shows how the needle and seat assembly contact the float lever arm.
Here is a center-hung float in a 4150 fuel bowl. The needle and seat assembly contact the top of the float, which hinges from a center-mounted location. This style provides more consistent float and fuel level in the bowl during both acceleration and lateral maneuvers.
The top of the main body has air-bleed orifices on both the primary and the secondary sides of a 4150- or 4500-series carb. The outboard brass holes are for the primary idle air bleeds.
Although the air bleeds on most carbs are press-fit, the 4500-series Dominators have slotted and replaceable air bleeds for fine-tuning. Using larger diameter orifices creates a leaner air/fuel mixture.
The locations of the air bleeds on the Dominator are the same as on other models. The inboard holes are the high-speed bleeds, and the outboard holes are for primary idle air bleed.
Center-hung float bowls have a needle and seat assembly located at the top center of the bowl. The fuel inlet is located on the passenger side of the bowl.
The hex nut engages to the needle and seat assembly via a rectangular engagement. A 5/8-inch wrench is required to drive the hex nut. Turning the hex nut clockwise raises the needle and turning the nut counterclockwise lowers the needle. Whenever the hex nut is removed, always inspect the gasket on the underside of the nut and replace if it is damaged. A slotted screw serves only to lock the nut in place.
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.
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.
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.
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.
With a metering plate, the fuel enters the metering holes at the bottom of the plate. At idle, fuel travels up from the metering holes along the angled “dogleg” passages, through the idle feed passages at the top of the doglegs, and then down through the vertical passages on each side of the plate to the idle transfer slot in the main body. When the fuel moves through the main circuit, it travels from the main metering holes upward through the inboard (slightly angled) passages where the fuel is emulsified by air from the main body. From there, it moves through the main body to the discharge nozzles and into the venturis.
Note the main metering holes at the bottom of this secondary metering plate.
Six 1/2-inch 8-32 screws secure the secondary metering plate to the main body on a 4160 carb. These screws are a clutch-head drive.
All fuel-bowl screws are thread size 12-24 and are available in two lengths. The 4150- and 4500-series primary and secondary fuel bowls, as well as the 4160 primary fuel bowl, use 2½-inch screws (top). The secondary fuel bowl on the 4160 carb (with the thinner metering plate) uses 1⅞-inch screws (bottom).
Fuel-bowl screws seal to the fuel bowls with fiber or nylon washers. Inspect these sealing washers carefully every time you remove or install bowl screws. If you plan to service the carburetor on a regular basis for tuning or general service, it’s a good idea to have new bowl screw washers on hand.
The underside of the baseplate has crossover channels, as well as small holes, called curb-idle discharge ports, which are in the throttle body ports under the throttle plates. Engine vacuum pulls a small amount of fuel from the discharge ports as part of the idle circuit.
The clutch-head screw drive looks like an hourglass. Although you can use a flat-blade screwdriver, you should, more correctly, use a clutch-head driver. A 5/32-inch clutch-head driver is available as a bit to attach to a drive handle or ratchet or as a dedicated driver with a full-size grip. The clutch-head driver engages the screw’s drive and provides secure removal or installation without the chance of damaging the screw’s drive. This full-size clutch-head driver is from NAPA (PN M-135).
The primary bowl has been removed from this 4160 carb. Its primary metering block is similar to that found on a 4150 model. Note the side-hung float used in 4160 models.
All Holley secondary metering plates are stamped with a one- or two-digit identification number in the center area. Production numbers, which can be ignored, are stamped at the center bottom. The “9” in the center identifies this plate as PN 134-9; its main metering holes are .067 inch and its idle holes are .031 inch. Metering plates are available in a wide range of primary and idle circuit sizes.
Shown here are the metering plate from a 4160 (left) and the metering block from a 4150 (right).
The 4160 (left) uses a thin metering plate between the secondary fuel bowl and main body; the 4150 (right) uses a metering block. This side-by-side-comparison clearly shows why a 4160 requires shorter screws for its secondary fuel bowl.
On the left is a two-stage power valve with small orifices. On the right is a power valve with a large window passage. The type with small holes is primarily designed to improve part-throttle economy on vehicles with heavy loads and isn’t the best choice for a performance application. The gasket style differs as well. The correct gasket must be used depending on style of power valve. If the gasket is incorrectly matched to the power valve, it leaks. The gasket with a concentric inside diameter is used with a power valve that has the larger window fuel opening. The gasket with three small tangs on the inside diameter seats properly onto the power valve with the series of small fuel orifices.
The power valve remains closed during idle and normal cruising. The power valve opens when intake manifold vacuum drops to a certain level and load increases to supply additional fuel.
Here the power valve has been installed in the metering block. A 1-inch wrench is required for removal or installation. The flats on the valve are very shallow, so make sure that the wrench is seated fully. A superior choice is a specialty power-valve wrench (see Chapter 4 for details). The opening vacuum of power valves is marked in inches. This valve is marked 6.5, meaning that it opens when manifold vacuum drops to 6.5 inches.
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 jets and power valve have been removed from this metering block. The thread size of the power-valve hole is 1/2 × 28.
Whenever replacing a power valve, it’s a good idea to install a new gasket. A power valve should be tightened to the metering block at 40 to 50 in-lbs. Avoid overtightening.
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.
A white fuel-resistant plastic fuel–bowl vent baffle extension, known as a “whistle,” aids in preventing fuel spillover during hard acceleration, deceleration, and cornering.
The main vent tubes are clearly visible on this 4150 carb.
This is a close-up of the primary vent tube.
