How a Simple Carburettor Works

A simple carburettor meters and mixes fuel with air by using a venturi to create a low-pressure region that draws fuel from a float-fed bowl through a jet, atomizes it into the airstream, and delivers the combustible mixture to the engine; the throttle controls airflow (thus engine speed), the choke enriches the mixture for cold starts, and dedicated circuits maintain fueling at idle and higher loads. This mechanical system relies on the Bernoulli principle and careful orifice sizing to keep the air–fuel ratio in an effective range across operating conditions.

The Physics Behind Carburation: Venturi Effect and Atomization

At the heart of a carburettor is a narrowed passage called a venturi. As air is drawn through the venturi by the engine’s intake strokes, it speeds up and its static pressure drops. This pressure differential between the float bowl (near atmospheric pressure) and the venturi throat pulls fuel through a calibrated jet.

Fuel emerging from the jet mixes with high-velocity air, breaking into fine droplets (atomization). Air bleeds and emulsion tubes introduce air into the fuel before it exits the jet, improving atomization and stabilizing mixture delivery over a range of flows. The goal is a combustible air–fuel ratio—around 14.7:1 by mass for gasoline at stoichiometric, slightly richer for power and colder conditions.

Key Components of a Simple Carburettor

A basic, single-venturi (single-barrel) carburettor uses a small set of mechanical parts to meter and mix fuel reliably. Below are the essential components and what each does.

• Air inlet and filter: Admits and cleans incoming air to protect jets and passages from debris.
• Venturi (choke tube): Narrows the airflow path, raising velocity and lowering pressure to draw fuel.
• Throttle plate (butterfly): A pivoting valve downstream of the venturi that regulates airflow and engine speed/load.
• Float chamber (bowl): A reservoir maintaining nearly constant fuel level via a float and needle valve.
• Float and needle valve: Open to admit fuel from the pump or tank; close when the correct level is reached.
• Main jet: A calibrated orifice supplying fuel to the venturi at moderate to high airflow.
• Emulsion tube and air bleed: Pre-mix air with fuel to improve atomization and flatten the fuel flow curve.
• Idle (pilot) circuit and mixture screw: Bypasses the nearly closed throttle to supply fuel–air at idle and very low throttle.
• Choke plate (air flap) or enrichment circuit: Temporarily enriches mixture for cold starting by restricting air or adding extra fuel.
• Vent/overflow: Keeps bowl pressure near atmospheric and prevents flooding damage.

Together, these parts establish pressure differences, meter fuel precisely through jets, and mix it with air so the engine receives an appropriate mixture at idle, cruise, and higher loads.

Operating Modes: What the Carburettor Does at Each Stage

Different passages and jets dominate at different throttle openings and temperatures. While the pure “textbook” simple carburettor has just main and idle circuits with a choke, many practical versions add refinements like air bleeds, and some larger automotive units add acceleration and power-enrichment features.

• Cold start: Closing the choke reduces intake air, increasing vacuum at the venturi and enriching the mixture to compensate for poor fuel vaporization in a cold engine.
• Idle: With the throttle nearly closed, airflow through the venturi is minimal. The idle circuit meters a pre-mixed fuel–air emulsion past the throttle edge, adjusted by the idle mixture screw.
• Off-idle/transition: As the throttle opens slightly, progression ports feed additional mixture until venturi flow is strong enough for the main jet to take over.
• Cruise/main metering: At moderate speeds and loads, the main jet and emulsion system supply fuel proportionally to airflow through the venturi.
• Acceleration (if equipped): Some carburettors include an accelerator pump to add a brief shot of fuel when the throttle is opened quickly, preventing a lean stumble.
• High load/enrichment (if equipped): Power valves or metering rods may enrich the mixture under heavy load to protect the engine and improve torque.
• Deceleration: High manifold vacuum with a mostly closed throttle leans the mixture; some systems add cut-off strategies to prevent afterfire and plug fouling.

The net effect is a largely self-compensating system where pressure differentials scale fuel flow with airflow, supplemented by special circuits to cover extremes like idle, cold start, and rapid throttle changes.

Mixture Control, Adjustment, and Tuning

Carburettors aim to deliver near-stoichiometric mixtures for clean cruising and slightly richer mixtures for power or cold operation. Adjustments tailor delivery to the engine, fuel, and environment.

• Idle speed screw: Sets the minimum throttle opening to control base idle rpm.
• Idle mixture screw: Fine-tunes the idle circuit’s fuel–air balance; adjust for highest, smoothest idle then slightly richer for stability.
• Main jet size: Larger jets enrich; smaller jets lean. Choose based on plug color, exhaust gas readings, or wideband O2 feedback.
• Float height: Alters fuel level in the bowl; too high risks flooding and richness, too low causes lean surging and starvation.
• Choke setting: Automatic chokes set by a thermostatic spring; manual chokes depend on operator input. Use only as long as needed.
• Altitude and temperature: Higher altitude and higher temperature reduce air density—mixture becomes richer; smaller jets or more air bleed may be needed.
• Fuel type: Ethanol blends can require slightly richer jetting and compatible elastomers to avoid swelling or corrosion.

Thoughtful tuning balances drivability, fuel economy, and engine health. Modern diagnostic tools (vacuum gauge, tachometer, wideband O2 sensor) make adjustments more precise.

Common Issues, Symptoms, and Maintenance

Because carburettors rely on small passages and precise levels, contamination, wear, or misadjustment can quickly degrade performance. Preventive care extends service life.

• Clogged jets or air bleeds: Causes lean misfire, surging, or flat spots; fix by ultrasonic cleaning and fresh filters.
• Sticking float or leaking needle valve: Leads to flooding, fuel smell, hard hot starts; replace worn parts and verify float height.
• Vacuum leaks (hoses, gaskets, throttle shaft wear): Produce erratic idle and lean conditions; locate with spray test or smoke machine.
• Carb icing: Moist, cool air can form ice near the venturi; mitigated by intake heat or anti-icing strategies.
• Perished seals and diaphragms: Ethanol and age can harden rubber; use ethanol-compatible components.
• Incorrect choke operation: Over-enrichment fouls plugs; under-enrichment causes cold hesitation.
• Fuel contamination: Rust or water from tanks clogs passages; add filtration and keep tanks clean.

Regular inspection, clean fuel, proper filtration, and occasional rebuilds (gaskets, needles, jets) keep a carburettor reliable.

Advantages and Limitations Compared to Fuel Injection

While still common in small engines and legacy vehicles, carburettors have largely been replaced in modern road engines by electronic fuel injection (EFI) for precision and emissions control.

• Advantages: Mechanical simplicity, low cost, easy field service, no high-pressure pump or ECU required.
• Limitations: Less precise mixture control, poorer cold-start and altitude compensation, higher emissions, and sensitivity to wear and contamination.
• Modern context: EFI offers closed-loop control with oxygen sensors, rapid transient response, and onboard diagnostics, meeting strict emissions and efficiency standards.

For applications prioritizing simplicity and cost—like lawn equipment, small motorcycles, generators—carburettors remain practical, but EFI dominates automotive and many powersports segments.

Step-by-Step Flow: From Tank to Cylinder

The following sequence traces how fuel and air move through a simple carburettor during normal operation.

1. Fuel enters the float bowl via the inlet; the float and needle valve maintain a set fuel level.
2. On an intake stroke, the engine lowers manifold pressure, pulling air through the carb throat and venturi.
3. Air speeds up in the venturi, pressure drops, and fuel flows from the bowl through the main jet/emulsion tube into the airstream.
4. Fuel atomizes, mixes with air, and passes the throttle plate into the intake manifold.
5. At idle, the idle circuit bypasses the nearly shut throttle to supply a controlled fuel–air emulsion.
6. For cold starts, the choke enriches the mixture until the engine warms enough for stable vaporization.

This flow adapts automatically with load and speed because higher airflow deepens the pressure drop at the venturi, proportionally increasing fuel delivery.

Summary

A simple carburettor uses the venturi effect to meter fuel from a float-controlled bowl into fast-moving intake air, with the throttle governing airflow and dedicated circuits handling idle and cold starts. Through calibrated jets, air bleeds, and basic mechanical controls, it delivers workable mixtures across operating conditions—an elegant, low-cost solution that, while superseded by fuel injection in most modern engines, remains effective in many small-engine and legacy applications.

What are the 7 circuits of a carburetor?

The circuits that comprise a carburetor are broken down into seven categories. They are: float, choke, idle, main metering, power enrichment, accelerator pump, and if applicable, secondary barrels.

How does a simple carburetor work?

The simplest carburetors work on Bernoulli’s principle: the static pressure of the intake air at the fuel entry point, which can be in a tube which is constant diameter, reduces at higher speeds compared with the pressure in the float chamber which is vented to ambient air pressure, with the pressure difference then …

What are the disadvantages of a simple carburetor?

Limitations of Simple Carburettor
Prone to icing in cold and humid conditions. Inconsistent performance at varying altitudes due to changes in air pressure. Limited adaptability to different engine requirements and fuel types.

How does a carburetor work step by step?

A carburetor works by the venturi effect, using the engine’s intake stroke to create a vacuum that draws air through a narrow tube (the venturi), which speeds up the air and drops its pressure. This lower pressure then sucks fuel from a float bowl through a small pipe into the airstream. The throttle valve controls the amount of this air-fuel mixture entering the engine to regulate power, while a choke valve restricts air for cold starts, creating a richer, fuel-heavy mixture.
 
Here’s a step-by-step breakdown of the process:

1. Engine Creates a Vacuum: Opens in new tabWhen an engine’s piston moves down during the intake stroke, it creates a vacuum that pulls air into the carburetor. 
2. Air Enters the Venturi: Opens in new tabThe incoming air passes through a venturi, a constricted section of the carburetor. 
3. Pressure Drop and Fuel Suction: Opens in new tabAs air passes through the narrow venturi, its speed increases, causing a drop in pressure (Bernoulli’s principle). This low-pressure area creates a suction effect. 
4. Fuel is Drawn from the Bowl: Opens in new tabFuel is stored in a float bowl, which maintains a constant fuel level via a float and needle valve. The pressure difference in the venturi pulls this fuel up through a small fuel nozzle or jet. 
5. Fuel and Air Mix: Opens in new tabThe fuel sprays into the airstream in the venturi, where it atomizes and mixes with the air to form a combustible mixture. 
6. Throttle Controls Power: Opens in new tabThe air-fuel mixture then flows through a throttle valve, which controls the amount of mixture entering the engine. Pressing the gas pedal opens the throttle wider, allowing more mixture and thus more power. 
7. Choke for Cold Starts: Opens in new tabFor a cold engine, a choke valve, located before the venturi, restricts the airflow. This enriches the fuel-air mixture, providing more fuel for easier starts. 
8. Mixture Enters the Engine: Opens in new tabThe atomized air-fuel mixture then travels into the engine’s combustion chamber to be ignited by the spark plug.