This article is part of a 6-part series, Part 2. It explains how the venturi creates vacuum to pull fuel and how the throttle valve controls engine speed. These principles matter because without precise fuel metering, an engine will not run correctly under different loads.
Bernoulli’s Principle and Atomization
By introducing a restriction (venturi) into the air horn, airflow is forced to accelerate. According to Bernoulli’s Principle, as the velocity of a fluid (air) increases, its static pressure decreases. This creates a localized vacuum at the venturi throat. A smaller venturi positioned inside a larger one creates a compounding effect. The outlet of the secondary venturi is placed at the point of highest vacuum in the primary venturi, multiplying the pressure drop to draw fuel more effectively at lower airspeeds. Liquid gasoline has high surface tension, causing it to exit nozzles in large droplets. By introducing a “bleed” (a small air stream) into the fuel nozzle, the fuel is partially atomized before it even enters the main airstream. This breaks the fuel into smaller particles that vaporize more readily. The system relies on the difference between atmospheric pressure acting on the fuel in the bowl and the partial vacuum in the venturi to “push” fuel through the discharge tube.
Component Relationships and Logic
The tip of the discharge tube is positioned at the center of the venturi throat. This ensures fuel enters the airstream at the point of maximum velocity and lowest pressure, facilitating immediate mixing. Jets act as the primary metering restriction within the fuel passageways. While the venturi determines the vacuum strength, the jet diameter determines the maximum volume of fuel that can be pulled by that vacuum. The internal bowl vent vents the bowl to the air horn to maintain pressure balance even if the air filter is restricted. The external vent (idle/anti-percolation) opens during idle or hot soak to vent fuel vapors to a charcoal canister, preventing engine-heat-induced “percolation” from flooding the intake.
Critical Tolerances and Specifications
Jet orifices are machined to extremely tight tolerances. Even microscopic changes in diameter significantly alter the air-fuel ratio. Vacuum measurements at the entry are low, for example 1 inch Hg. At the venturi throat, maximum vacuum occurs, for example 3 inches Hg, due to peak velocity. At the exit, recovered pressure or reduced vacuum occurs, for example 2 inches Hg, as air expands and slows. The discharge nozzle must be slightly higher than the fuel level in the bowl to prevent fuel from “siphoning” out when the engine is not running.
Assembly and Housing Logic
Venturis are typically cast into the carburetor body or air horn. In high-performance applications, they may be removable inserts to allow for tuning the airflow capacity of the unit. The sequential atomization process is as follows. First, air enters the air horn and accelerates through the primary venturi. Second, compounded vacuum is created in the secondary venturi. Third, air bleeds mix with fuel in the discharge tube. Fourth, metered fuel exits the nozzle and undergoes final atomization in the high-velocity airstream. Passageways and jet housings are typically made of non-corrosive zinc or aluminum alloys to maintain the integrity of calibrated fuel paths.
Throttle Control and Final Metering
Airflow Regulation and Volumetric Efficiency
The throttle valve acts as the primary regulator of engine speed by controlling the volume of air-fuel mixture allowed to enter the intake manifold. When the throttle is in a closed or restricted position, it creates a high vacuum in the intake manifold below the valve, while atmospheric pressure remains above it. As the valve opens, this vacuum drops, and the volume of air (and subsequently fuel drawn by the venturi) increases. The air bleed system introduces air into the fuel stream before it exits the discharge nozzle. This creates a “pre-mix” that reduces the surface tension of the fuel, ensuring it shatters into a fine mist upon entering the main venturi airstream.
Component Relationships and Logic
The throttle valve is located at the base of the air horn, downstream of the venturi. By regulating the total airflow, the throttle indirectly controls the vacuum strength at the venturi. Without this relationship, the engine would operate only at a single, unmodulated speed. The throttle valve is a circular metal plate mounted on a shaft. It is mechanically connected via linkage to the accelerator pedal. The degree of shaft rotation is directly proportional to the volume of air-fuel mixture reaching the cylinders. Jets are often pressed or screwed into the lower ends of main discharge tubes. They act as the final metering orifice to ensure that the volume of fuel traveling through the passageway does not exceed the engine’s requirements at wide-open throttle.
Critical Tolerances and Specifications
In the fully closed position, the throttle valve must seal the air horn almost completely. Any gap or “light” visible around the edges can lead to a high or erratic idle speed (vacuum leak). High-precision jets are constructed of brass or stainless steel to prevent orifice erosion over time, which would otherwise enlarge the opening and richen the mixture. The tip of the main discharge tube must remain at a specific height above the fuel bowl level to prevent “nozzle drip” caused by hydrostatic pressure when the engine is static.
Local Shop Note:
You know, this takes me back to a conversation I had with a mechanic over on NY-342 in Calcium, N.Y. We were at an ATTS seminar, and he was telling me about a sedan that came in with a complaint that it would idle rough and stall when coming to a stop, but it ran fine at cruise. The customer had already rebuilt the carburetor and replaced the idle mixture screws. Still idled rough.
He checked the float level — within spec. Checked the idle mixture screws — set correctly. Then he started looking at the throttle plates. He noticed the primary throttle plate wasn’t closing fully — there was a tiny gap between the plate and the bore. That gap was letting unmetered air bypass the idle circuit, leaning out the mixture at idle. The engine was compensating by pulling extra fuel through the idle feed restrictors, which was making the idle unstable and causing the stall.
He loosened the throttle plate screws, centered the plate in the bore, and retightened them. The plate sealed properly after that, and the idle smoothed out with no stalling.
The lesson for you guys is: the throttle plate has to seal completely at idle. If it’s misaligned or binding, it acts like a vacuum leak and throws off the idle circuit. Always check the throttle plate alignment before you start adjusting mixture screws or rebuilding the carburetor. Sometimes the problem is mechanical, not fuel.
Assembly and Housing Logic
The throttle body features a thick mounting flange that serves as the structural foundation of the carburetor. It must be torqued evenly to the intake manifold to prevent warpage of the throttle shaft bore. Metering passageways are drilled into the carburetor casting and often sealed with lead plugs or threaded inserts. These paths are designed with specific diameters to manage fuel velocity and prevent air pockets. Bleeds are strategically located in the upper portion of the air horn to draw clean, filtered air from above the venturi, ensuring the pre-atomization process does not introduce contaminants into the fuel circuit.
The key takeaway is that the venturi shape and throttle position work together to control fuel delivery and engine speed. Proceed to Part 3.