Which intake manifold should you choose? Holley and other manufacturers offer a wide array of single- and multiple-carb manifolds. In basic terms, if you want low-end torque at lower engine RPM, consider a dual-plane intake manifold. Single-plane manifolds tend to favor upper-end power at higher engine speeds. A very broad generalization is to select a dual-plane manifold for the street and a single-plane manifold for higher-RPM operation where low-end torque isn’t as critical.
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:
LEARN MORE ABOUT THIS BOOK HERE
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Never fire a fresh engine without first pre-oiling. A pre-oiler connected to shop air forces oil through all of the oil passages, ensuring that the valvetrain and all bearings are lubed prior to starting.
Torque Requirements
Prior to mounting the carburetor on the intake manifold, you should have previously checked both the carb mounting base and the mounting surface of the manifold for flatness. Although either screws or studs may be used to mount a carburetor, the best choice by far is to use a set of carburetor mounting studs.
If you use bolts, you must be very careful not to use bolts that are too long because they can bottom out inside the blind thread holes in the manifold, preventing carb-to-manifold sealing. Also, using studs on the manifold provides an easy positioning guide for the carb.
When installing studs, install them finger-tight. Do not double-nut studs and torque them into the manifold. This can easily create a splayed condition. Tightening the nuts provides the needed clamping force.
After placing the carburetor mounting gasket in place on the intake manifold, position the carburetor onto the studs. Install flat washers and nuts finger-tight. Using an inch-pound torque wrench, tighten the nuts in a crisscross pattern to evenly distribute the clamping force. Do not overtighten the nuts, as this can easily warp the carb’s mounting base.
Always refer to the carburetor’s instruction sheet for recommended torque, which is typically around 60 in-lbs. Seriously, it’s very easy to accidentally warp a carb base, so take the time to evenly tighten the nuts. Don’t guess.
Linkage Setup
Setting up and adjusting the carburetor linkage is relatively straightforward. In the relaxed position (throttle pedal not depressed), the linkage should be adjusted so that the primary throttle shaft is not being moved or influenced by the throttle pedal.
With the engine off, manually move the carb throttle lever fully to the wide-open position and verify that the linkage doesn’t prevent wide-open operation. In addition, have a helper work the throttle pedal slowly to wide open while you observe carburetor lever movement. The linkage adjustment should allow WOT, without applying additional pressure against the carb throttle lever at wide open. Perform all of these checks with the engine off. Also, while a helper works the throttle pedal, check for smooth operation from closed to wide open. Any stiff or binding issues must be addressed.
If the carburetor has mechanical secondaries, verify that the secondary throttle plates are opening. Generally, once the primary throttle plates open approximately 40 to 45 degrees, the mechanical secondary plates should begin to open. The opening rate of the secondaries should be a bit faster than that of the primaries, with both primary and secondary plates fully open at the same time under WOT. The most critical aspect, from a safety standpoint, is that there is no binding or sticking during any phase of throttle movement. The linkage, whether it involves rods or a cable, should connect the carb throttle lever in as straight and horizontal a position as possible. Avoid severe angles.
Engine Pre-Start Requirements
If the engine is new or freshly rebuilt, you must pre-oil it by either pressurizing the oil circuit with a pre-oil canister or removing the distributor (if so equipped) and turning the oil pump driveshaft until the oil system is fully pressurized. A mechanical oil pressure gauge should be connected (even if this is to be temporary) to verify oil pressure. If the engine design does not use an oil pump that is shaft driven from the distributor (if the oil pump is crank driven, as on GM LS engines, for example), the only way to pressurize the oil system is by using a pressurized oil tank. Do not crank the engine to build oil pressure. An oil film should already be present at all main, rod, and cam bearings, as well as at the entire valvetrain prior to cranking the engine. Again, this applies only to an engine that is new and/or freshly rebuilt.
If the engine is equipped with a new flat-tappet camshaft rather than a roller cam, a break-in period is required for the cam and lifters. If this is the case, it’s wise (but not mandatory) to first use a known, good carburetor to ensure an immediate startup and running during camshaft break-in. After the cam break-in period, you can then install the new or rebuilt carb and continue with tuning and adjustments. You don’t want to find yourself fiddling with a carb problem while you’re trying to seat the new cam. You also need to make sure a few other important procedures have been performed. The fuel tank must be clean and have sufficient fuel to run the engine for a while. Be sure that the fuel offers a high-enough octane level for the particular engine. Also, be sure that the fuel system pump is in good condition and that a fresh fuel filter is installed. The vehicle should be parked in an area with good ventilation (outdoors or in a shop/garage with good ventilation). The engine’s cooling system must be in good condition and the coolant level must be at specification.
Emergency Precautions
If you plan to run the engine for an extended period while tuning or adjusting the carburetor and/ or engine timing, it’s a good idea to place a large floor fan in front of the radiator to minimize the possibility of engine overheating. Make sure that the vehicle is parked in a safe condition (in park or neutral with the parking brake engaged). Placing wheel chocks in front of and behind the tires is a good idea.
Whenever starting an engine with a new fuel system for the first time, the concern for a fuel leak and the potential of a fire cannot be ignored. Keep a fresh, fully charged fire extinguisher, a selection of towels, and a bucket of water handy (a shop towel can smother a small fire in the case of a carburetor backfire). Think seriously about safety before starting the engine. Be prepared for a problem.
If possible, park the vehicle in such a manner that it can be pushed out of your garage in case of a fire emergency. You likely want to position the vehicle with its rear facing your bay door, so that you can open the door and allow exhaust fumes to exit. Just make sure that it is not obstructed, including by other vehicles, so that the car can be pushed out if a fire gets out of hand. If an emergency presents itself, at least you have a better chance of preventing a house fire.
Wear gloves (fire-retardant Nomex or Kevlar gloves are a good choice) and eye protection in case of a fire and in case a pressurized fuel line squirts fuel into the air. Wearing Nomex or Kevlar sleeves is also a good idea. These are available from any race supply source. They serve as arm protectors from the wrist to near the elbow. In combination with Kevlar or Nomex gloves, they protect your hands and arms from heat and sharp edges if you need to reach into a hot engine bay.
And, especially if you’re working alone, be sure to keep a cell phone handy just in case you need to call 911.
Vacuum Leaks
If you hear a whistling sound and/or the idle is rough, you may have an engine vacuum leak. A vacuum leak, if left unattended, causes the engine to run lean.
Carefully retighten the carburetor mounting nuts to 60 to 80 in-lbs, in a crisscross pattern. Carefully check the intake manifold bolts for proper torque. Refer to your intake manifold instructions or factory manual. With an aluminum intake manifold, your specified torque may be 25 to 35 ft-lbs, but don’t guess. Follow the specs for your application.
Smoke machines are made by a number of manufacturers. They are simple to use and can save lots of time by quickly locating a leak source.
Smoke machines utilize a non-toxic and material-safe fluid, such as baby oil. The machine is connected to a 12-volt source such as the vehicle battery, along with compressed air. The fluid is gently heated and pushed into the engine at any source to check for fluid or vacuum leaks.
You can use a smoke machine to locate any number of mystery vacuum leaks.
Check for open ports such as vacuum hose connections at the carb and manifold. Make sure that all ports are connected or plugged as required for your application. If you have trouble locating a vacuum leak, try spraying propane, carb cleaner, or WD-40 along the intake manifold-to-head mating locations and along the base of the carb. Spray slowly and along a specific route. If the engine suddenly smooths out, you’ve located the leak area.
Here, a vacuum leak was quickly found at a distributor vacuum advance hose connection.
Another method to locate a leak (and the one I prefer) is to use a smoke machine. This is a portable unit that holds a supply of non-damaging liquid such as baby oil. The unit heats the liquid and turns it into a light smoke. A hose connects the machine to the engine. You can connect the end of the hose to a variety of locations, such as a vacuum port on the manifold. With the machine running and the engine off, look for smoke escaping from the engine. The escape location indicates the vacuum leak. You can use a UV light to help observe the smoke.
It’s rare, but a vacuum leak could occur because of a crack or pinhole in a manifold casting. Vacuum leaks can also occur as the result of a cracked or cut vacuum hose or poor vacuum hose connection. A smoke machine helps to pinpoint the exact location of a leak.
A smoke machine is also handy for locating other leaks, such as fuel or oil leaks. Fuel leaks are located by introducing smoke at any point in the fuel system, such as the fuel tank, or by disconnecting a fuel line or hose and inserting the smoke machine hose into the circuit. Allow sufficient time for the smoke to travel. The smoke won’t harm anything and it won’t stain any surfaces as would spraying WD-40 or carb cleaner.
Improper tightening of the intake manifold can easily result in vacuum leaks. Always use a torque wrench to tighten all intake manifold fasteners. If some bolts are difficult to access and you don’t have room for a socket wrench, you can still torque to desired value using a torque wrench adapter.
Throttle Operation
Before firing the engine, carefully operate the throttle by hand to be sure that the throttle does not bind at any point, to avoid a sticking throttle, and free-revving the engine. Make sure that the throttle closes completely when fully released, and make sure that the throttle opens all the way to WOT. Don’t depend only on manual checks at the carb and linkage. Have a helper work the throttle pedal while you observe throttle movement, from closed to wide open. Verify that there are no sticking or binding points, and that the throttle moves smoothly from closed to wide open. Linkage adjustments should be made now, before attempting to start the engine.
When you’re checking and adjusting the carburetor on the engine, never hold any small items in your hands such as screws, nuts, etc. This will help you to avoid dropping foreign objects into the carb.
Smoke that escapes from a vacuum or fluid source is usually easy to see, but a hand-held UV light aids in spotting the leak, especially in a confined or dark area.
A throttle bind may be caused by an overtightened and/or unevenly tightened baseplate. Relax all four mounting fasteners that secure the carb to the manifold and retighten evenly in a crisscross pattern to the proper torque specification, which is usually 60 to 80 in-lbs. Excessive and uneven tightening of the mounting fasteners can distort the baseplate, resulting in either a vacuum leak or throttle bind, or both.
Also make sure that the carburetor mounting gasket clears the throttle plates. If the gasket has four individual holes, and the holes are too small for the throttle plates, the plates can dig into the gasket material and bind or jam the throttle plates. Depending on the intake manifold, use of a gasket with a single large square bore opening avoids this concern.
Carefully check throttle operation to make sure that you have no binding issues, and that the throttle is able to move freely from fully closed to WOT.
Do not overtighten carburetor mounting nuts. Most applications call for 60 to 80 in-lbs of torque when mounting a carb to an intake manifold.
Proper tightening requires the use of a torque wrench. It’s too easy, especially for the novice, to overtighten and unevenly tighten, which can easily result in vacuum leaks and a warped carb baseplate.
Adjusting the operation of the accelerator pump lever to “time” pump activation in relation to the carburetor throttle movement is rather easy. With the engine off, make the adjustment at the “override” spring and screw between the pump lever and carb lever. With the throttle relaxed in the idle position, use an open-end wrench to hold the screw’s locking nut (at the base of the screw) steady while turning the screw head.
Before mounting the carburetor to the intake manifold, closely inspect the mating surfaces on both the carb baseplate and the manifold to make sure that both are clean and free of any contaminants that compromise gasket sealing.
Tighten the screw, which compresses the spring, until you have freeplay between the screw and pump-lever arm. Then, slowly back off the screw, just to the point where freeplay is eliminated. Next, move the throttle to the wide-open position. While holding at WOT, insert a feeler gauge between the pump arm and the adjuster screw. You should be able to move the pump arm an additional .015 inch. This clearance prevents the pump arm from bottoming out and placing undue stress on the arm.
Fuel Pressure
Especially if you are running an electric fuel pump, be sure to add a fuel pressure regulator. Although a fuel-injected system requires relatively high fuel pressure (depending on the application, this could be 40 to 60 psi); a carburetor requires only about 6 to 8 pounds of pressure. Running excessive fuel pressure into a carburetor overrides the needle and seat and has the potential for damaging various gaskets. Be sure to choose an electric pump that is designed for use with the lower pressure required for a carburetor. The regulator should be adjustable within a range of 6 to 8 pounds. Once the engine starts, adjust the fuel pressure accordingly.
When multiple carburetors are installed, a fuel distribution “log” allows fuel to enter from the pump and is then distributed to each carburetor. This example is on a Ford flathead, using a fabricated billet aluminum fuel log. Note the pressure gauge mounted on top of the log.
When running nitrous injection, a separate fuel supply must be delivered to the nitrous circuit. Note the individual pressure regulators for the carb and for the nitrous system’s fuel.
A fuel pressure gauge plumbed into the feed makes it easy to monitor pressure. Mounting a fuel pressure gauge can be done in a number of ways. Fuel feed tubes/bars/rails are available with 1/8-inch NPT holes, as are some fuel plumbing fittings to allow easy installation of a pressure gauge. The pressure gauge must be installed after the regulator, between the regulator and the carburetor inlet(s).
Whenever installing any NPT fitting (tapered threads) to a fuel line, the threads require a fuel-resistant sealant. Avoid using Teflon tape because threads from it can enter the fuel system and clog critical ports within the carburetor. Instead, use a Teflon pipe sealant paste. Don’t get carried away with the amount. Use only enough paste to coat the threads.
Carburetor Plumbing Techniques
Typically, a fuel feed system has 3/8- or 1/2-inch inside-diameter hose and/or tubing. When using –AN plumbing, the equivalent is size –6 or –8. The –AN hose size is easy to understand when you realize what the dash numbers represent. The –AN system is based on increments of 1/16 inch. A –6 size is essentially 6/16 (or 3/8) inch. A –8 size is 8/16 (or 1/2) inch. Depending on the volume of fuel required for the particular engine, a –6 size is usually adequate for street engines; a heavier-breathing performance engine may be better suited to –8 plumbing. When in doubt, going bigger to a –8 won’t hurt.
Whether you opt for flexible hose or hard tubing for the majority of your plumbing, remember that you must have a flexible hose between the engine and chassis, to allow for engine rock during idle, acceleration, or deceleration. It’s always a good idea to install a fuel filter between the tank and fuel pump, regardless of whether you’re using a mechanical or electric pump. However, you must install a filter between the fuel pump and carburetor to prevent contamination from entering the carb.
If plumbing with flexible hose, you have the option of running slip-on hose secured with clamps, or –AN hose with threaded-connection fittings and hose ends. –AN plumbing looks professional and it offers a much more secure connection. In addition, –AN hose is offered in a number of compositions (rubber lined with stainless steel exterior braiding, multi-layer heavy-duty hoses, rubber-lined with black composite fabric braiding, etc.) High-performance –AN hose exterior is designed to protect the hose from abrasion.
If running hard-line tubing, common materials include aluminum and stainless steel. When custom bending is required, aluminum fuel tubing is easier to work with and is also more manageable for flaring. Hard-line tubing is easily fitted with –AN threaded connections. The hardware attached to the tubing includes a ferrule and a tube nut.
A traditional –AN hose end has a nipple tube that engages into the hose. As you thread the sections together, a compression fit secures the connection.
The –AN plumbing hardware has a 37-degree seat for sealing. No additional thread-sealing compound should be necessary. Just make sure that the threads are lightly lubricated to avoid galling.
AN plumbing provides a much more secure and robust fluid connection in contrast to a slip-on hose that’s secured with a clamp. A wide range of fitting and hose-end shapes are available to suit any plumbing routing, including but not limited to straight, 45-degree (shown), 90-degree, and 180 degree.
Slip the tube nut over the tube first, followed by the ferrule. Using a flaring tool, create a single flare at the end of the tube. The tube nut is now ready to engage onto a –AN male fitting.
When plumbing from the fuel pump to the carburetor, you need to feed both fuel bowls if the carb is a double-pumper. This is most easily accomplished by using a fuel inlet feed (also called a fuel log or fuel rail, depending on design). It has a horizontal run of hose or tube, with individual fuel feeds for the primary and secondary bowls. These are available pre-assembled and ready to install to the carb. Depending on side clearance, a variety of inlets are available, from short to long, to clear any potential obstruction such as a choke mechanism.
Although a fuel injection system requires relatively high fuel pressure, perhaps in the 40- to 50-psi range, a carburetor requires only about 6 to 8 psi. If fuel pressure is excessive, it overrides the needle and seat and floods the carb.
If you’re running an electric fuel pump, be sure to obtain a pump that’s rated for use with a carb. Because the pump may be able to provide higher pressure, you should always install an adjustable fuel pressure regulator between the pump and carb. Even if you’re running a mechanical fuel pump, using a pressure regulator is still a good idea. In conjunction with an adjustable regulator, you should install a fuel pressure gauge, which is located between the regulator and carb.
Specialty aluminum wrenches for the range of –AN sizes are readily available from several sources. If you have –AN aluminum plumbing on your vehicle, you need a set of these tools.
Whenever you deal with aluminum –AN fittings and hose ends, always use an aluminum wrench to avoid damaging the surface.
Fuel-compatible high-performance hose is available in a variety of construction, including rubber-lined stainless braid, high-temperature synthetic braid, and rubber/Kevlar.
An option to traditional –AN hose-to-hose end construction is the crimp-collar construction, which provides a secure connection with reduced weight. A special crimping machine is required to assemble this style.
An option to traditional –AN hose-to-hose end construction is the crimp-collar construction, which provides a secure connection with reduced weight. A special crimping machine is required to assemble this style.
A common method of feeding a double-pumper carb is with a fuel feed created from –AN stand-offs, –AN fittings, and flexible braided fuel hose. You may assemble the fuel feed yourself or purchase it ready-to-install, based on the center-to-center fuel inlet distance of the specific carburetor model.
Fuel inlet fittings that accept –AN hose ends are available in short “stock” length.
Extended-inlet fittings are available in a range of lengths.
A simple way to feed both bowls of a Double Pumper carb is with a three-way distribution block.
Many carburetor fuel inlet logs have 1/8-inch-NPT-threaded ports to allow the installation of a pressure gauge. With a gauge in place, you can adjust and verify your fuel pressure. When selecting a pressure gauge, the better choice is a liquid-filled gauge. It dampens the gauge needle to allow more accurate observation of fuel pressure, compensating for engine vibration. A non-dampened gauge, especially on an engine with a radical camshaft, allows the gauge needle to bounce erratically, making it difficult to pinpoint the adjusted pressure.
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:
LEARN MORE ABOUT THIS BOOK HERE
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This fuel inlet plumbing has an adjustable fuel-pressure regulator mounted before the fuel rail, with a fuel pressure gauge mounted on the fuel rail. The gauge can be mounted anywhere along the fuel inlet rail, as long as it’s between the regulator and the carb.
Carburetor fuel inlet fittings are available in several styles. If the hex is very close to the carb body, you may not be able to use a line wrench. If using an open-end wrench, take your time and make sure that the wrench is fully and firmly seated on the fitting to avoid slipping and damaging the fitting.
Note the extended fuel feed standoffs on this fuel rail. Various lengths of fuel inlet fittings/extensions are available, allowing you to mount the fuel rail as far away from the carb as necessary to clear any obstructions. Manufacturers including Aeroquip, Fragola, Russell, Earls, Goodridge, XRP, and others offer a wide range of carburetor fuel inlet fittings to suit any application.
Spacers made of a composite or phenolic material can help to reduce heat-soak at the carburetor. If a spacer is used, longer carburetor mounting studs are required.
Note the milled air reliefs on this carb spacer. They serve to further aid in heat dissipation.
An aluminum carburetor spacer not only slightly extends the air column but also serves to dissipate heat. When a spacer is used, a gasket is required between the manifold flange and spacer as well as between the spacer and carb.
Dual Carbs
When running twin carburetors, a distribution block or fuel manifold is required to feed both carbs from a single primary feed line. In addition, throttle linkage must be carefully set up and adjusted to allow each carburetor’s throttle plates to operate in unison or progressively as needed. Linkage rods featuring threaded rods equipped with radiused rod ends provide reduced friction and smooth operation.
Whether the carburetors are mounted in-line or side-to-side (as with a cross-ram setup) is often dictated by the configuration of the intake manifold and available clearance. The operating rods should be oriented in as horizontal a manner as possible. Avoid severe angles that can alter lever ratios and that can potentially cause a binding situation. The throttle linkage base must be firmly anchored, usually to the intake manifold.
The vertical rod that connects the base to the carb rods should be as vertical as possible. Plan on spending a bit of time to design the linkage rod system. The objective is to attain smooth operation, with full throttle closure at rest and full WOT without any binding or excess friction.
Dyno Testing Your Setup
Not everyone has access to an engine dyno, but running the engine on a test cell and gathering data can help to adjust the carburetor and tune it for maximum power before installing the engine. Bear in mind that as you tune a carb (and engine timing as well) for maximum horsepower and torque, power and torque are developed at various engine speeds. The dyno gets you into the ballpark. Once the engine is installed to the vehicle, further tweaking may be required to compensate for vehicle weight, tire diameter, operational speeds and loads, etc. Gathering engine dyno data allows you to know what the engine is capable of and provides you with a quantitative reference.
Linkage rods are available with radius rod ends for friction-free pivoting. Rods should have right-hand threads at one end and left-hand threads at the other end, to allow fine-tuning of length adjustment without the need to disconnect one end. When the length has been properly adjusted, be sure to tighten the jam nut at each end to prevent the rod from loosening and changing length.
Setting up a dual-carb linkage on a sideways-mounted carb requires careful attention to rod length and length adjustment. Radius rod ends provide low-friction for all pivot points. A primary horizontal shaft connects the actuator rods to each carb.
After installing the carb, you inevitably have to spend some time calibrating it for a particular engine and set of conditions. Always have your tools handy before starting the engine. You may need to adjust the idle stop, idle-mixture screws, float level, and/ or re-tighten manifold or carb mounting fasteners.
An engine dyno allows you to fully control and monitor the engine while verifying operation and testing/ tuning for maximum horsepower and torque. Remember that an engine dyno provides data only on the engine. Additional tuning and tweaking may be needed after the engine is installed into a vehicle. A chassis dyno allows you to monitor and tune while gathering both engine data and power applied to the drive wheels.
Pre-Oiling a Fresh Engine
It may seem out of place to discuss engine pre-oiling in a carburetor book, but it’s extremely important to note that if you are dealing with a new engine that’s never been run before, or an engine that’s been sitting in storage for a long period of time, it’s critical to make sure that the bearings and valvetrain are properly lubricated prior to firing the engine for the first time.
Do not disconnect the coil(s) and crank the engine to establish oil pressure. The proper method is to prime the oiling circuit without trying to crank the engine via the starter. This can be done in several ways, depending on the type of engine. If the engine has a distributor and an oil pump that’s driven by the distributor shaft-to-oil pump shaft, you can remove the distributor and drive the oil pump with a heavy-duty electric drill or pneumatic drill, using a drive adapter that engages the oil pump input shaft.
If the engine has a crankshaft-driven oil pump (this refers to many late-model engine designs, such as GM’s LS), you can’t run the pump with a drill. In this case, using a pressurized engine pre-oiler canister is the answer. Actually, this type of delivery system works for any type of engine.
The canister holds engine oil and is pressurized with compressed air. A hose connects the canister to the engine block, at an oil entry port. Once the canister’s valve is opened, oil is pushed into the engine, delivering oil to all bearings and the valvetrain. Regardless of the approach you use, remove the valve covers to expose the rockers. Pressurize the oiling system until you see oil exiting at all of the rocker locations. In some cases, this may take a few minutes.
Although vehicle weight, rear axle ratio, and tire diameter all factor into final engine tuning, being able to quantify the engine prior to vehicle installation is invaluable. Rather than guessing at horsepower and torque, at what RPM points these occur, an engine dyno provides this information and allows you tune and adjust the fuel and spark, providing an informed baseline. This particular engine pulled 1,014 hp initially, and managed to pull 1,115 hp and almost 900 ft-lbs of torque, after a wee bit of carb tuning and ignition curve adjustments.
It’s a good idea to manually rotate the crankshaft slowly during the oil priming process so that at some point the oil feed holes in the bearings align directly to the oil passages in the main saddles and in some camshafts. During oil pressurization, slowly rotate the crank two full turns. The crank rotation may not be absolutely necessary, but it’s a good idea.
Setting and adjusting the engine’s ignition timing goes hand in hand with carburetor adjustment when pursuing optimum idle and maximum power.
Prior to firing the engine, double-check all linkage attachments to prevent linkage, screws, or nuts from vibrating off.
When you know that oil has been distributed throughout the oil circuit, you can then turn your attention to firing the engine and checking and adjusting the carburetor. Engine pre-oiler canisters are available from various sources. Some are steel (basically the same construction as a propane tank); others are made of aluminum. The aluminum units prevent potential rust scale buildup inside the tank that might be present in an improperly stored steel unit.
Air Cleaners
Unless you’re running the engine in a dust-free environment (such as marine use), you need a filter that prevents contaminants from entering the engine. With that said, pay attention to the air cleaner base (the bottom section). Make sure that you retain at least 3/8-inch clearance between the air cleaner base and the carburetor’s vent tubes. This is necessary whether the vent tubes are square cut or angle cut. If the base is too close to the vent tubes, it can mess up the reference pressures for the carburetor’s bowls.
An air cleaner provides more than simply filtering out airborne debris. An air cleaner with a filter element also protects against potential fires caused by backfiring.
Velocity stacks are sometimes chosen simply because they look cool. Regardless of the appearance factor, a velocity stack can effectively lengthen the flow path of the intake manifold runners on a manifold with isolated runners (rather than a dual-plane manifold), potentially improving cylinder filling. Basically, a velocity stack can help to tune the length of the air column that enters the engine. A velocity stack, because of its vertical air intake design, needs sufficient space above the top bell of the stack, so the application should have no hood or if a hood is on the vehicle, you should maintain at least 2 inches of space between the hood and the stack.
Many variables come into play, but using velocity stacks has the potential to improve airflow and power, although the improvement may be minimal. The only reason to run a velocity stack on a street engine is for the looks. If you desire a velocity stack for a V-8 engine, a good source is a performance marine distributor because they are popular for high-performance boat applications.
Float Height
Place a rag under the fuel bowl to catch any potential fuel spill. If the fuel bowl is equipped with a brass sight plug, remove the plug and its gasket. With the vehicle on a level surface and with the engine idling, hold the needle and seat float-adjustment hex steady with a 5/8-inch box wrench and loosen the lock screw with a broad flat-blade screwdriver. Then, rotate the hex slowly to adjust float level. Turn the hex counter-clockwise to raise the fuel level or clockwise to lower it until the fuel level is at the bottom of the sight hole. Take your time and adjust it in small, slow increments; wait a few seconds until the fuel level stabilizes.
When installing an air cleaner lid, don’t get carried away. If you use excessive force, you might place undue stress on the carburetor’s main body casting.
Individual velocity stacks provide isolated airflow paths for each carburetor. The length of the stack extends the airflow column of the intake manifold runners.
Hold the hex steady with the 5/8-inch wrench and tighten the lock screw. Rev the engine a few times and then allow it to return to idle to verify that fuel level hasn’t changed. Reinstall the brass sight plug and its gasket. Some Holley carbs have a fixed-glass sight window that allows you to observe the fuel level without having to remove a sight plug.
Idle Mixture
Initially, turn the idle-mixture screws in fully and gently (clockwise), until the screw stops, then back it out by one turn. Connect a vacuum gauge to the engine at a manifold vacuum source. With the engine running, adjust the mixture screw on one side of the metering block until you achieve the highest vacuum reading. Go to the opposite side and repeat the process. Then go back to the first side and repeat, then to the opposite side and repeat again. Do this until you have achieved the highest stable vacuum reading.
If the carb is equipped with idle-mixture screws on the secondary metering block, perform the same procedure.
Generally speaking, an acceptable idle mixture should be obtained with the mixture screws adjusted to somewhere between 1/2 and 2 turns out from fully closed. Once all of the mixture screws are adjusted, re-adjust the idle speed to the desired RPM. You may need to re-check idle-mixture adjustments. The engine should be warmed to normal operating temperature to make sure that the fast-idle cam was released before final adjustments.
When adjusting needle and seat height, hold the nut stationary with a 5/8-inch wrench while loosening the locking screw. Then turn the wrench to raise or lower the needle and seat. Once adjusted, hold the nut steady and tighten the locking screws.
When initially adjusting the idle-mixture screws on each side of the metering block, begin by turning the screw all the way in until it gently bottoms out, then turn the screw counterclockwise 11⁄2 turns. This provides a starting point for further adjustment as the engine runs.
A needle screw on each side of the primary metering block adjusts idle mixture. This is called a two-corner idle. Some carbs may have idle screws on the secondary metering block as well; this is known as a four-corner idle system.
On “non-emissions” carbs, turn the screw clockwise to lean the mixture and counterclockwise to richen the mixture. As the screw is turned inward (clockwise), the air/fuel flow to the curb-idle discharge port is reduced, leaning the mixture. If the screw is seated fully inward, this flow is stopped.
With the engine off, turn each idle-mixture screw inward (clockwise) until the screw lightly bottoms out. Then turn the screw counterclockwise by 11 ⁄2 turns. Do this to all idle-mixture screws. This provides a starting point.
Connect a vacuum gauge to a full manifold vacuum port. If the vehicle is equipped with a manual transmission, leave the transmission in neutral with the parking brake on. If equipped with an automatic transmission, set the parking brake and, once the engine has started, place the transmission in Park. Start the engine and allow it to warm up to operating temperature. Adjust each mixture screw to achieve the highest vacuum reading. Adjust each screw by the same amount.
Once the idle mixture has been adjusted, you may then adjust curb-idle speed by turning the idle-speed adjuster screw on the carb linkage. Once idle speed has been adjusted, re-check all mixture screws, again with the goal of achieving the highest vacuum reading.
Engine Bogs
If the engine “bogs” upon acceleration, chances are that the accelerator pump shot isn’t sufficient. Curing this may involve moving to a larger discharge nozzle, a larger capacity accelerator pump, or both. Changing to a camshaft with more duration can require a greater pump shot.
Depending on other variables, such as vehicle weight and stall speed of an automatic transmission torque converter, moving to a smaller discharge nozzle may help the existing accelerator pump do its job.
Again, always make only one change at a time, otherwise you won’t know which change helped or hindered performance. As engine load increases (using a lighter flywheel, moving to a lower-RPM torque converter, etc.), you may be able to reduce discharge nozzle size. As the engine load increases (moving to a higher stall converter or a heavier flywheel), you may have to increase discharge nozzle size, and possibly go to a larger accelerator pump.
Experiment with discharge nozzle size. When you obtain a crisp throttle response, record the nozzle size, then move up an additional .002 inch and test the response. Chances are that you are in the right ballpark.
Check clearance between the accelerator pump lever and the adjuster screw. You should have about .015 to .020 inch of clearance between the pump lever and the adjuster when the throttle is moved to WOT. Check the clearance between the accelerator pump lever and the pump lever cam. If you lowered the idle speed, it moves the cam a bit farther away from the lever, which delays the pump shot. Adjust the plastic cam so that there is zero clearance between the pump actuating lever and the cam.
Power Valve
You need a vacuum gauge to monitor intake manifold vacuum and to determine when the power valve opens. The correct power valve for the application should provide a slightly higher opening point than the highest manifold vacuum to protect the engine from a lean condition. However, if manifold vacuum is very low at idle or part-throttle, a lower opening point may be needed to obtain a power valve that doesn’t open until the engine needs the added fuel.
For example, if the carb is equipped with a 65 power valve, and the engine vacuum sometimes drops to, say, 5 Hg at idle, installing a 40 power valve that opens at 4.0 Hg may help. For WOT use, such as racing, always use a power valve that opens slightly higher than the highest manifold vacuum to avoid a lean condition.
You need to be aware of the relationship between main jets and the power valve. All too often, “tuning” for performance is done by simply changing jets to maximize power at WOT. If jet size is increased and you’re not happy with WOT power, you may be allowing too much fuel to enter at part throttle, even when the power valve circuit is closed at high manifold vacuum. You’re better off tuning the power-valve circuit first (by changing the power valve) to maximize efficiency at part throttle, and then changing jets to optimize power at WOT.
Tuning Vacuum Secondary Operation
Vacuum-operated secondary systems are intended to open the secondaries when the engine demands additional fuel and air. Ideally the transition should be smooth and unnoticed by the driver. Drivers may think that the secondaries aren’t working properly if they can’t “feel” an abrupt kick in the pants. If a sudden surge is felt, it indicates that there was a split-second transition when the engine stumbled and “hiccupped” before surging forward. The opening rate of the vacuum secondary system can be tailored by changing the spring inside the vacuum housing.
Holley offers a kit (PN 20-13) that contains several color-coded springs. If the vehicle is light and the engine is large, a lighter spring can be used to accommodate a faster engine speed increase. Conversely, a heavier vehicle with a smaller engine can benefit from using a stiffer spring to avoid a stumble upon acceleration. Just remember that a vacuum secondary system, when tuned properly, should provide a seamless transition when the secondaries open.
Written by Mike Mavrigian and Posted with Permission of CarTechBooks
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