How Fuel Travels from the Carburetor to the Cylinder

Fuel reaches the cylinder as an air–fuel mixture: the carburetor meters and atomizes fuel into the incoming air, the piston’s intake stroke creates a pressure drop that draws this mixture through the intake manifold, and it then passes the open intake valve (or reed/transfer ports on two‑strokes) into the cylinder. This process relies on vacuum, precise metering through jets, and coordinated valve or port timing to deliver a combustible charge for ignition.

The Core Physics: Vacuum, Venturi, and Atomization

In a carburetor, airflow speeds up through a narrowed section called a venturi, lowering pressure and pulling fuel from the float bowl through metering circuits (jets). This fuel mixes with air and is partially atomized into tiny droplets. The throttle plate controls airflow volume, while the choke enriches the mixture for cold starts. As the piston descends on the intake stroke, manifold pressure drops below atmospheric, and the pressure differential moves the air–fuel mixture from the carburetor, through the intake runner, and into the engine. Heat from the manifold and engine helps vaporize fuel further, improving mixture quality before compression and ignition.

The Four-Stroke Path

In four-stroke engines, the intake, compression, power, and exhaust strokes are distinct. The air–fuel mixture’s journey from carb to cylinder predominantly occurs during the intake stroke, when the intake valve opens and cylinder pressure falls.

The list below outlines the step-by-step path of the air–fuel mixture in a typical four-stroke, carbureted engine.

1. Fuel delivery to carb: Fuel flows from the tank via gravity or a pump into the carburetor’s float bowl, where a float and needle valve set the fuel level.
2. Metering and mixing: Air accelerates through the venturi, drawing fuel through idle, transition, and main jets (and often an emulsion tube with air bleeds) to form the mixture.
3. Throttle passage: The mixture passes the throttle plate; its opening angle controls airflow and thus how much mixture proceeds into the intake manifold runner.
4. Intake stroke into the cylinder: As the piston descends with the intake valve open, manifold vacuum draws the mixture past the valve curtain area into the cylinder.
5. Charge preparation: Some droplets wet the manifold walls and valve; heat promotes vaporization and mixture homogenization before the valve closes for compression.

Taken together, these stages ensure a metered, combustible mixture reaches the cylinder precisely when the intake valve and piston motion create favorable pressure conditions.

The Two-Stroke Path

Two-stroke engines use the crankcase as a pump and rely on ports (and often reed or rotary valves) instead of cam-driven intake valves. The path includes a detour through the crankcase before the charge is transferred to the cylinder.

The following list breaks down how a two-stroke moves mixture from the carburetor into the combustion chamber.

1. Crankcase intake: As the piston rises, crankcase pressure drops; reeds (or a rotary valve) open, drawing the air–fuel mixture through the carburetor into the crankcase.
2. Crankcase compression: As the piston descends, it pressurizes the crankcase; the reeds close to prevent backflow.
3. Transfer to the cylinder: When transfer ports uncover, the pressurized mixture flows from the crankcase through these ports into the cylinder, scavenging out exhaust gases.
4. Compression and ignition: The piston rises, closing the ports and compressing the fresh charge prior to ignition.

This loop-scavenged process both fills the cylinder and purges exhaust, with mixture motion shaped by port design to minimize losses out the exhaust port.

Key Components That Make It Happen

Several parts must work in concert to move fuel from the carburetor to the cylinder with the correct ratio and timing. Below is a quick map of the main players and what they do.

• Fuel tank and pump (or gravity feed): Supply fuel to the carb.
• Filter and lines: Keep debris out and maintain consistent flow.
• Float bowl, float, and needle valve: Regulate fuel level for stable metering.
• Jets and circuits (pilot/idle, transition, main; sometimes power valve): Meter fuel across operating ranges.
• Emulsion tube and air bleeds: Pre-mix fuel with air for finer atomization.
• Throttle plate or slide (and, in CV carbs, a diaphragm): Control airflow and mixture volume.
• Choke or enrichener: Temporarily increases fuel for cold starts.
• Intake manifold/runner and gaskets/boots: Channel mixture and maintain vacuum integrity.
• Intake valve and ports (four-stroke) or reeds/rotary valve and transfer ports (two-stroke): Time and route mixture entry to the cylinder.

When these components are clean, sealed, and correctly adjusted, the engine receives a consistent, combustible charge across idle, acceleration, cruise, and high-load conditions.

Why Fuel Sometimes Doesn’t Reach the Cylinder

If an engine won’t start or runs lean, the obstruction is often somewhere between the tank and the intake valve/ports. The items below outline common culprits and what they imply.

• Fuel supply issues: Empty tank, failed pump, clogged filter, or collapsed fuel line restrict flow to the carb.
• Float/needle faults: Stuck float or worn needle prevents proper bowl level, starving jets.
• Clogged jets or varnish: Deposits from old fuel block idle or main circuits, especially after storage.
• Vacuum leaks: Cracked manifold boots, gaskets, or hoses dilute mixture and reduce draw through the carb.
• Choke/enrichener faults: Maladjustment or failure causes hard cold starts and lean running.
• Airflow problems: Severely restricted or water/oil-soaked air filter upsets carb metering.
• Two-stroke reed damage or four-stroke valve timing issues: Chipped reeds or incorrect valve timing/compression hinder intake charge.
• Carb icing: In cool, humid conditions, ice in the venturi reduces airflow and fuel draw; manifold heat or anti-icing helps.
• Poor fuel quality: Stale fuel or ethanol phase separation changes volatility and metering.

Systematically checking supply, metering, sealing, and mechanical timing usually isolates the cause of mixture starvation.

Small Details That Matter

Atomization and Vaporization

Smaller droplets and adequate heat improve burning. Emulsion tubes, booster venturis, and heated manifolds enhance vaporization; cold, short trips often worsen wall wetting and drivability.

Mixture Control Across Operating Modes

Idle and off-idle circuits handle low throttle; the main jet handles mid-to-high load; accelerator pumps cover transient throttle changes; some carbs add power enrichment for wide-open throttle.

Orientation and Manifold Design

Downdraft carbs benefit from gravity and more uniform distribution; sidedraft setups suit tight spaces. Runner length and plenum design influence mixture speed, cylinder-to-cylinder balance, and throttle response.

Summary

Fuel leaves the carburetor as a metered, partially atomized air–fuel mixture driven by manifold vacuum. In four-strokes, the mixture passes the throttle, intake runner, and open intake valve into the cylinder on the intake stroke; in two-strokes, it first fills and is pressurized in the crankcase, then flows through transfer ports into the cylinder. Proper metering, sealing, and timing ensure a reliable charge reaches the cylinder for efficient combustion.

How does fuel get into the cylinder?

In gasoline engines, the air and fuel are pre-mixed before being sucked into the cylinder. On the other hand, diesel engines use fuel injectors to spray fuel into the cylinder.

What delivers fuel to the cylinder?

A fuel pump draws the fuel from the tank through fuel lines and delivers it through a fuel filter to either a carburetor or fuel injector, then delivers it to the cylinder chamber for combustion.

What forces fuel from the carburetor into the cylinder?

The Venturi effect draws fuel from the carburetor bowl due to pressure differential and atomizes it this mixing it with air as a (if you will) more of a mist. The cylinder downstroke draws the fuel/air mixture into the cylinder where it is compressed and ignited.

How does fuel get from the carburetor to the engine?

Operating principle
Air from the atmosphere enters the carburetor (usually via an air cleaner), has fuel added within the carburetor, passes into the inlet manifold, then through the inlet valve(s), and finally into the combustion chamber.