A carburetor’s inlet system features a fuel bowl, where a specific amount of liquid fuel is stored. A float inside the bowl features a tapered needle that engages into a seat orifice. The adjustment level of the float, along with fuel pressure, maintains the correct fuel level in the bowl. As the fuel level drops, the float drops; this moves the needle away from its seat, allowing fuel to enter the bowl. When the fuel level rises to its adjusted setting, the float rises, moving the needle toward its seat. This simple float-activated needle and seat system maintains the required level of fuel within the bowl, providing the adequate amount of fuel to be delivered to the carburetor’s metering system.
Fuel inlet pressure, the pressure created by the fuel pump, directly affects float level. If too much fuel pressure is present, the float rises, causing the engine to run too rich, with excess fuel spilling/venting into the carburetor’s air inlet. If fuel pressure is too low, the float drops and lowers the bowl fuel level, resulting in decreased fuel delivery to the main jets, promoting a lean condition.
A fuel-injected system requires a high fuel pressure, often in the 30- to 40-psi range, in order to provide instant fuel delivery when the injectors open. On the other hand, a carbureted fuel system requires a relatively low fuel pressure, commonly in the 4- to 6-psi range. In a carbureted system, too-high fuel pressure overwhelms the float, causing the float to rise too far. Whenever an electric fuel pump is used, you need to pay attention to its rated pressure output. If the pump is rated to produce in excess of about 6 psi, a fuel pressure regulator must be installed in the fuel line. The best setup is to use an adjustable pressure regulator along with a pressure gauge, to allow you to adjust and verify fuel pressure.
A 4-barrel, or secondary-type, carburetor offers more fuel than a 2-barrel when the secondaries are opened. The primary side is used for light throttle demands and the secondaries deliver maximum air/fuel delivery for heavy acceleration.
From the bowl area, fuel flows through main jets, which control the flow of fuel into the carburetor’s metering system. Main jet sizes are based on venturi size, atmospheric pressures, and ambient operating temperatures. The venturi provides a point of constriction for incoming air, acted upon as the down-stroke of the pistons creates a vacuum signal. As air runs through the venturi, it increases speed and then creates a pressure drop as it exits the venturi. This pressure drop promotes fuel from the bowl into this vacuum pull, pulling fuel through the discharge nozzle in the boost venturi. The mixture of fuel with air atomizes the fuel.
A passage in the carburetor applies vacuum to a power valve located between the bowl and metering system. At engine idle speed, vacuum is highest. At idle, high vacuum keeps the diaphragm in the power valve closed. As the throttle opens during increased demand, vacuum drops, which allows the spring inside the power valve to overcome vacuum and opens the diaphragm. This allows added fuel to flow through the valve, richening the fuel mixture to accommodate the increase in throttle-opening air demand. In other words, when you open the throttle, the power valve provides added fuel.
An accelerator pump, located at the bottom of the bowl, serves as a mechanically operated fuel injector that supplies an extra shot of fuel when the throttle is opened suddenly/quickly. This added shot of fuel reduces or eliminates the chance of a stumble or lag upon sudden throttle opening. The accelerator pump features a lever that is actuated by the throttle linkage.
A 4-barrel, or secondary-type, carburetor broadens the power output potential by adding more air and fuel delivery when engine demands require more delivery. A secondary-type carburetor is essentially two carburetors in a single package. Both the primary and secondary sides feature their own metering systems. The primary side is used for light throttle demands; the secondary side is designed to operate when additional or maximum air/fuel delivery is required. The secondary side activates either mechanically, via the carburetor throttle linkage, or by vacuum, utilizing a diaphragm that opens the metering circuit based on engine vacuum. A vacuum-operated secondary-type carburetor is more forgiving because the secondaries open only as required according to the engine’s vacuum signal. Therefore, this type of carburetor offers more latitude, allowing the use of a slightly larger CFM rating in contrast to a mechanically operated secondary that is controlled by the driver’s throttle control.
Choosing Carburetor Size
Many are tempted to run a bigger carburetor, in terms of CFM rating, assuming that this automatically results in added power. Some even choose a larger carburetor simply for bragging rights. Like so many other engine components, bigger isn’t necessarily better. The volume of the carburetor must be matched to the engine’s needs. You need to match the carburetor according to the engine’s volumetric requirements.
Volumetric efficiency (VE) refers to the engine’s ability to breathe. VE represents a ratio of the weight of the incoming ambient air to the theoretical volume of air that the engine can consume at the anticipated engine RPM at which it makes maximum torque. VE is expressed as a ratio of these two factors. A stock, low-performance engine likely features about 80-percent VE at its maximum torque range, while a modified, better breathing performance engine may feature a VE in the 85- to 90-percent range. If you refer to carburetor CFM size for a given engine displacement and peak-torque RPM, you can determine what size carburetor is appropriate, based on a theoretical 100-percent VE. You then multiply peak RPM by the engine’s VE in order to determine what the actual carburetor size should be.
This chart aids in selecting a carburetor size for anticipated wide-open throttle (WOT) at the engine’s lowest RPM. (Photo Courtesy Holley Performance Products)
This chart aids in carburetor selection based on engine-operating RPM. For example, a 400-ci engine theoretically requires about 800 cfm when operating at about 6,800 rpm. Factors such as cylinder head, intake manifold, and cam specifications are variables that come into play. The chart provides a good starting point for carburetor selection. (Photo Courtesy Holley Performance Products)
The higher the engine’s VE, the bigger the carburetor it can utilize. VE can be increased by increasing the engine’s breathing, which can entail using a camshaft with more duration, choosing a more efficient and freer-flowing intake manifold, improving the exhaust system flow, cylinder head porting, and reducing the engine’s parasitic losses by accurizing all clearances, achieving proper cylinder bore surfaces finish, enhanced rotating assembly balancing, and taking advantage of specialized anti-friction and oil drain-back coatings, etc.
In the most basic terms, the larger the engine displacement and the higher the engine speed, the more air it can consume; therefore, the larger the carb can be.
Simple Carb Formula
The following formula may be used to roughly determine carburetor size based on engine displacement and maximum engine speed:
Maximum Carb CFM = (CI ÷ 2) × (maximum RPM ÷ 1,728)
Where:
CFM = cubic feet per minute
CI = cubic inches of displacement
RPM = revolutions per minute (engine