Selecting the right carburetor for a given engine and vehicle combination can seem confusing. The first order of business is to choose the appropriate airflow capacity, or CFM (cubic feet per minute), especially if your goal is to maximize horsepower. Remember the old adage, “bigger is not always better.” If a carburetor is too small for a given application, it doesn’t provide enough air and fuel for the engine to reach its potential. By the same token, if the carb size (CFM rating) is too great, the engine will likely bog, which also prevents maximum performance.
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Next, consider the application, or how the vehicle will be operated. If the vehicle is intended strictly for the street, a carburetor equipped with vacuum secondary operation is almost always better than a double-pumper carb with mechanical secondaries. Although a mechanical-secondary carburetor can be tuned for street operation, it’s simply easier to go with one that’s equipped with a vacuum secondary system; the secondaries open only when the engine’s load requires their operation.
You must decide whether you need a choke for your application. If the vehicle is intended for the street, even if driven only occasionally, you need a choke. Restricting airflow to the primary side of the carb aids in engine startup, especially in cooler temperatures. If no choke is present, you need to adjust the throttle during startup, and then allow the engine time to warm before venturing onto public roads.
For a vehicle that is used only on the track (drag strip, road course, oval track, off-road competition, etc.), easy startup when the engine is cold isn’t that critical, and you usually have adequate time for the engine to warm to operating temperature before making a run on the strip or before the first lap on a course. For a competition vehicle, eliminating a choke mechanism reduces weight, simplifies the makeup of the carb, and eliminates any potential for primary airflow restriction if the choke plate sticks or binds.
If you like the idea of having a choke, consider the type of choke you prefer. A mechanical choke allows the driver to control the position of the choke plate via a manually operated cable. It provides total control; but the driver must be alert enough to open the choke when the engine no longer requires primary-side air restriction. Properly adjusted, an electric choke automatically restricts airflow and produces a richer fuel condition. After the ignition is energized, the choke plate automatically begins to open as the engine temperature rises. The choke plate moves to the full-open position as the engine warms to operating temperature and no longer requires this temporary rich condition.
Different carburetor models have varying numbers and locations of vacuum ports. If you’re running power brakes, you need a full-manifold vacuum port to provide vacuum assist to the power brake booster. This may involve a port at the rear base of the carburetor to provide full manifold vacuum; the intake manifold itself may have a threaded port at the rear wall of the plenum, just below the carburetor-mounting flange.
Selecting Carburetor Size
Carburetor “size” (or CFM capability) is selected with a host of variables in mind, not just for the engine but also for the vehicle, drivetrain, and vehicle usage. If the vehicle is equipped with an automatic transmission, for example, consider the torque converter stall speed, which represents the lowest engine RPM at which WOT is applied. If the vehicle is equipped with a manual transmission, consider the lowest engine RPM that you expect to use at WOT.
The relationship between the lowest engine RPM that you expect to use at WOT and engine displacement roughly determines the most appropriate carburetor size.
The relationship between the lowest engine RPM that you expect to use at WOT and engine displacement roughly determines the most appropriate carburetor size.
Bear in mind that this applies when selecting a carburetor with mechanical secondary operation. If you plan to use a vacuum-secondary carb, you can always increase CFM; the secondary operation is enabled according to engine demand.
The carburetor must be matched to the application. Although it may be tempting to purchase a larger carburetor than the engine requires, you need to avoid it because it won’t yield more horsepower. Most want maximum horsepower with fuel efficiency, and if your engine receives more fuel than it can effectively burn, the unburned fuel is pumped out the exhaust port and makes the spark plugs wet. Therefore, gas mileage suffers.
With a carb size that is too large, absolute top-end power might improve, but off-the-line and cruising mid-range acceleration can suffer (bogging). If you err on the side of caution by choosing a slightly smaller carb, you may sacrifice a bit of top-end power but your low-end and mid-range acceleration improves.
A formula to use as a base reference follows. Bear in mind that this formula assumes that the engine operates at 100-percent efficiency, which doesn’t happen in the real world.
Carburetor CFM = engine ci ÷ 2 x maximum RPM ÷ 1,728
1,728 = factor to convert cubic inches (ci) to cubic feet (cf)
This formula indicates half of the specified engine displacement because a four-cycle engine intakes once with each crank revolution. If the engine size is 455 ci and the maximum expected engine speed is 6,000 rpm, CFM is 789.93 (455 ÷ 2 x 6,000 ÷ 1,728). In this example, the engine prefers the closest-available carburetor size of 800 cfm. Because most engines (assuming that it is properly built and is operating at its best) have a volumetric efficiency in the 80- to 95-percent range, a slightly smaller carb (in the range of 650 to 750 cfm) is a more realistic choice for this engine.
Before you try to determine the ideal carburetor size for a given application, you need to have an idea of the engine’s volumetric efficiency (VE), which is an indication of the engine’s ability to breathe. The better the engine breathes, the higher the VE. This is expressed as a ratio of the air mass (weight of the air) that the engine intakes compared to the air mass that the engine’s displacement theoretically ingests if there were no losses. VE is fairly low at idle and varies according to engine speed. VE should be considered according to the engine RPM at which the engine realistically operates.
An engine cannot operate at 100-percent VE in the real world; it simply isn’t possible. To maximize VE, you need to verify some things. The engine must be in good operating condition and not worn out. This means that the compression ratio must be close to new, oil pressure is within acceptable ranges, all bearings operate well, and no other significant problems are evident.
A typical low-performance engine may have a compression ratio in the range of 70- to 80-percent VE at the engine’s maximum torque level. A typical high-performance engine that offers increased breathing may have a VE in the range of 80 to 85 percent at maximum torque. A top-level racing engine may have a VE in the range of 90 to 95 percent at maximum torque RPM.
The variables of engine parameters, including tuning the intake and exhaust system, cylinder head porting, camshaft profile, etc. all work in combination to increase VE. You can consult one of Holley’s carb selection charts. Based on 100-percent VE, if the chart indicates that the best size is, for example, 700 cfm, if the engine has around 85-percent VE. So, in theory, a better choice for that engine might be a carb size of 600 cfm.
Forced induction (such as supercharging) usually requires about 40- to 50-percent more carburetor capacity compared to a naturally aspirated engine. (Holley now offers a line of carburetors designed specifically for supercharged applications.)
Charts and formulas aside, consider moving to a bigger carb when displacement increases, when higher RPMs are to be used, when engine compression is higher, when mechanical ignition advance increases, when higher-ratio drive gears are used, and as vehicle weight is decreased. A smaller carb suits conditions when engine compression is low, the vehicle is heavier, when torque converter stall speed (on an automatic transmission) is very low, and when the drive gear ratio is numerically lower.
Also, keep in mind that a given carb (or carb tuning) used during an engine dyno session doesn’t necessarily perform the same when the engine is in the vehicle. The engine dyno run is merely a starting point; additional tuning (and possibly a move to a different carburetor) is potentially necessary once the vehicle is on the track (or road).
The throttle may be actuated via the accelerator pedal by either a rod linkage system or a flexible throttle cable, depending on the specific vehicle or the direction chosen by a custom vehicle builder. If a throttle cable is employed, it must be routed as smoothly as possible, with no sharp bends or kinks. A kinked cable or extreme bends can easily result in throttle binding, and this can lead to a sticking throttle at high engine speeds, which is an obvious safety hazard, to say the least. Also, make sure that a throttle cable is kept far enough away from any extreme heat source, such as the exhaust manifold or headers.
Carefully examine any throttle cable before installation to verify that it moves easily and freely within its casing. Whenever a throttle cable’s condition is questionable, it’s best to just replace it with a new cable. A throttle cable’s casing must be anchored at a convenient point to prevent the entire cable assembly from moving during acceleration and deceleration. Only the internal cable must be able to move, not the exterior sheathing.
A cable retention bracket should be mounted either on the driverside rear carburetor mounting bolt at the intake manifold or on the intake manifold itself. A variety of cable mounting brackets is readily available to suit any carb/manifold application.
If the throttle actuation is handled by a linkage rod system (1960s and 1970s Mopars are a good example), the linkage must be assembled correctly and all pivot points must be able to move freely without binding. You must pay strict attention to the assembly procedure, especially when dealing with a linkage rod assembly (rather than a throttle cable); make sure that the rods do not interfere with any nearby surfaces.
Carefully monitor the linkage system, from closed throttle to WOT. Make sure that the linkage does not touch any other surfaces that will cause the linkage to bind or rub. This includes the cylinder head, valvecover, rear of the block, any nearby screws, bolt heads, hoses, wiring, etc.
Pay attention to linkage rod geometry. Linkage rods (or cable connections) should be set up to push or pull the throttle lever in a straight line. When viewed from the side, they should be parallel with the base of the carburetor. When viewed from above, the linkage rods or cable should be straight and parallel with the side of the carburetor. Misalignment (when the rod or cable is at an offset angle) can result in excessive side loading and can require more force to operate the throttle. This can lead to premature wear of the carburetor’s throttle shaft.
Adjust the linkage to prevent “over-pull” or “over-push” at the throttle lever. At WOT, the linkage should be adjusted so that the force applied does not try to push the throttle lever beyond its WOT point. If this limit-stop is not adjusted properly, a heavy foot applied to the throttle pedal exerts excess force at the throttle lever, potentially causing premature throttle shaft wear and possible distortion of the throttle lever.
Do not rely solely on the carburetor’s built-in throttle-lever return spring for throttle closing. An additional return spring must be used to ensure that the throttle closes in real time relative to the operation of the throttle pedal. Dual springs with an inner and outer spring are popular choices. The lighter inner spring applies a light tension that assists throttle closing; the heavier outer spring provides added muscle to pull the throttle lever closed.
Installation of the spring is similar to the linkage installation. It should provide a straight pull, with the spring installed as close to parallel to the carb baseplate as is possible. With the engine off, test the throttle while monitoring the spring to make sure that the spring does not rub, interfere, or hang up at any point from closed to WOT. Carefully adjust and test all throttle lever connections, including cable and/or linkage, and the return spring(s) before firing the engine. Never start the engine until you are absolutely certain that you have no binding issues, and that the throttle closes easily. Performing these initial inspections with the engine running poses the very real risk of the engine running wild at WOT, with the potential for a disastrous engine failure.
Carburetors that are equipped with a vacuum port to accommodate a vacuum-advance distributor have a small-diameter vacuum fitting (requiring a 1/8-inch hose) located on the passenger’s side of the primary metering block, just above the mixture screw. If you have a distributor with vacuum advance, connect the distributor vacuum hose to this timed port on the carburetor to eliminate spark advance at closed throttle. A port in the throttle bore is exposed to vacuum as the throttle plate moves past the port, usually at slightly off-idle, providing the vacuum signal to the distributor.
A large-diameter full-manifold vacuum port that requires a 3/8-inch vacuum hose (to provide a vacuum signal to a power brake booster) is located at the rear of the carburetor baseplate (depending on the carburetor model). If this is not included on your carb, full-manifold vacuum can be accessed at the rear of the intake manifold plenum.
If neither the carb nor the intake manifold has a full-vacuum port at the rear, you can drill and tap the intake manifold plenum to accept a 3/8-inch NPT fitting to provide a full-vacuum signal to the power brake booster. Of course, this should be performed with the intake manifold removed from the engine. All current Holley carburetors, with the exception of the 4500 Dominator series, already have this large-diameter full-manifold vacuum fitting at the rear of the baseplate.
Some carb models also have two additional full-time vacuum port fittings at the front of the baseplate. They also include a 3/8-inch fitting for PCV hose connection and a 1/8-inch fitting. These provide full vacuum for operation of the air cleaner, air pump diverter valve, A/C, cruise control, and, if the vehicle is so equipped, temperature-sensing valve.
Any vacuum port fitting on the carburetor that is not needed must be closed off with a rubber vacuum cap. Carburetors such as the 4150 (including Avenger and Ultra) have four vacuum fittings. These are for the full-time rear power brake booster, the full-time front PCV, the full-time front accessories, and the timed vacuum on the passenger’s side of the primary metering block for the vacuum distributor advance. The PCV vacuum fitting is located on the front (primary side) of the baseplate because the primary usually flows more air, and also so that the crankcase vapors pulled by the PCV are shared evenly between the two primary throttle bores.
If your carb is equipped with an electric choke system, adjustments are available that allow you to set the opening and closing time/duration of the choke plate. Initially, the choke should be open enough to provide throttle response without engine stalling. If the choke comes off too early (doesn’t stay on long enough), rotate the choke’s thermostatic choke cap (the black housing) counterclockwise. If the choke comes off too late (stays on too long), rotate the choke cap clockwise. Always rotate the cap one notch at a time and test the operation until it is satisfactory. Be sure to re-tighten the three choke cap hold-down screws.
A choke that comes off too early can result in engine stalling, surging, stumbling, or backfiring when the engine is cold. A choke that comes off too late causes an over-rich condition, resulting in rough idle, poor driveability, lousy fuel mileage, and black smoke emitted from the exhaust.
The choke mechanism has a fast-idle cam that is connected to the choke thermostat. The cam has a series of steps that allow idle to step down during warm-up until the choke plate is fully open; it contacts an adjustable screw on a lever that connects to the primary throttle plates. The throttle stops against this plastic cam to raise engine speed during choke operation; holding the choke at a greater throttle angle produces a higher idle speed. The higher idle speed aids in atomizing the fuel for cold-engine operation. Before starting a cold engine, hold the throttle pedal to the floor; the fast-idle cam holds the primary throttle plates open more than usual, to raise idle RPM during initial start and warm-up. The fast-idle speed is set at the factory, and should provide 1,500 to 1,600 rpm. If idle speed requires adjustment, simply adjust the fast-idle screw.
With the engine off, allow the engine to completely cool to ambient temperature, making sure that the choke plate has fully closed. With the engine off, manually open the carb to WOT and hold it there. This exposes the fast-idle screw (located behind and to the lower rear of the choke housing). Using a 1/4-inch open-end wrench, turn the screw clockwise to increase RPM or counterclockwise to decrease RPM.
Close the throttle, start the engine, and check engine speed. Re-adjust until you achieve approximately 1,500 to 1,600 rpm at cold fast idle. As the engine warms, the cam drops onto its second step as the vacuum choke pull-off opens the choke slightly. The cam eventually drops to its last step when the choke plate opens fully.
If the carburetor is equipped with an electric choke, connect the choke cap’s positive terminal directly to a steady 12-volt power source that is activated by the ignition switch. Do not connect it to the ignition coil. A suitable 12-volt connection includes leads or wiring for vehicle accessories, such as the windshield wiper motor. However, do not connect the choke’s power lead to the original equipment electric choke source, because it might not be a 12-volt source.
Written by Mike Mavrigian and Posted with Permission of CarTechBooks