What a Carburetor Does—and Why It Still Matters

A carburetor’s function is to meter, mix, and atomize fuel with air in the correct ratio for combustion, then regulate engine speed and power via the throttle and enrichment circuits. In practice, it uses pressure differences (the venturi effect) to draw fuel into the incoming air stream, adjusts that mixture for idle, acceleration, and cold starts, and keeps fuel at a constant level so the engine runs smoothly under changing loads. While modern cars use electronic fuel injection, carburetors remain common in small engines, classic vehicles, and some aircraft.

How a Carburetor Works

At the heart of the carburetor is a narrowed passage called a venturi. As air flows through the venturi, its velocity increases and pressure drops, drawing fuel from a reservoir into the airstream. A throttle plate downstream controls airflow, which in turn controls engine power and speed. Additional circuits fine-tune the mixture for different operating conditions.

The Venturi Effect and Fuel Metering

The venturi creates a localized low-pressure region. Fuel is metered through jets into this area, mixing with air. The goal is to maintain an air–fuel ratio near stoichiometric for gasoline (about 14.7:1 by mass) under cruise, richer during acceleration and high load for power and cooling, and richer at cold start to compensate for poor fuel vaporization.

Core Components and Their Roles

The following components work together to keep the air–fuel mixture consistent and the engine responsive across conditions. Understanding each part clarifies how a carburetor fulfills its function.

• Float bowl and float/needle valve: Maintains a constant fuel level so metering remains predictable.
• Main jet and emulsion tube/air bleeds: Meters fuel for midrange and high-load operation and helps atomize it.
• Idle (pilot) jet and mixture screw: Supplies fuel at closed or nearly closed throttle for stable idle.
• Throttle plate (butterfly): Regulates airflow, controlling engine power and speed.
• Choke plate or enrichment circuit: Temporarily enriches mixture for cold starts.
• Accelerator pump: Adds a brief fuel squirt to prevent stumble when the throttle opens quickly.
• Power valve or secondary circuit (on some designs): Enriches mixture under heavy load or opens additional barrels for more airflow.
• Constant-velocity (CV) diaphragm slide (on many motorcycles): Smooths throttle response by adjusting air velocity over the jet.

Together, these parts meter, mix, and atomize fuel while adapting to idle, cruising, acceleration, and cold-start needs without electronic controls.

Operating Modes

Carburetors transition through several modes as the engine runs. The sequence below outlines how mixture control shifts with driver demand and engine state.

1. Cold start: The choke reduces incoming air (or an enrichment valve adds fuel), creating a richer mixture to aid ignition.
2. Idle and off-idle: The throttle is nearly closed; the idle/pilot circuit supplies fuel through small ports for smooth running.
3. Part-throttle cruise: The main jet and emulsion system take over, targeting efficient, near-stoichiometric combustion.
4. Acceleration: The accelerator pump (or a vacuum-operated enrichment) momentarily adds fuel to avoid a lean stumble.
5. High load/high RPM: Additional enrichment via power valves or secondary barrels supports power and engine cooling.
6. Deceleration: Reduced throttle cuts airflow; some systems add air to mitigate backfiring and excess fuel draw.

This progression allows a mechanically controlled device to handle a wide range of demands, approximating optimal mixture without sensors or software.

Why Carburetors Gave Way to Fuel Injection

Electronic fuel injection largely replaced carburetors in automobiles because it offers precise metering under all conditions, better fuel economy, lower emissions, altitude compensation via sensors, easier cold starts, and improved transient response. That said, carburetors persist in lawn equipment, generators, small motorcycles, classic cars, and some piston aircraft due to simplicity, repairability, and lower cost.

Maintenance, Tuning, and Common Symptoms

Carburetors require periodic tuning and cleaning, especially with today’s ethanol-blended fuels that can leave deposits and absorb moisture.

• Cleaning: Varnish or debris in jets causes lean running, hesitation, or stalling; ultrasonic cleaning and fresh gaskets often restore function.
• Float level adjustment: Incorrect levels lead to rich (too high) or lean (too low) mixtures across the range.
• Idle mixture and speed: Set with the engine warm; adjust for peak, smooth idle without excessive CO emissions.
• Choke verification: Ensure the choke fully engages cold and fully opens warm to prevent flooding or poor drivability.
• Vacuum leaks: Cracked hoses or gaskets introduce unmetered air, causing high idle or lean misfires.
• Ethanol considerations: Use stabilizer for seasonal storage; drain bowls in long layups to prevent gum and corrosion.

Regular attention to these areas preserves the carburetor’s ability to meter and mix fuel accurately, maintaining performance and reliability.

Special Note: Aviation and Small Engines

Many light-aircraft piston engines still use carburetors; pilots manage mixture manually and monitor for carburetor icing, which can restrict airflow and cut power. In small engines, simple fixed-jet carburetors remain standard because they are inexpensive and easy to service, even if less efficient than modern injection systems.

Summary

A carburetor’s essential function is to meter, mix, and atomize fuel with air, delivering the right ratio for combustion while controlling engine power through the throttle and auxiliary circuits. It accomplishes this mechanically using the venturi effect, jets, and valves to adapt to idle, cruising, acceleration, and cold starts. Although largely supplanted by fuel injection in cars, carburetors remain relevant where simplicity, cost, and ease of service are paramount.