Engine Fuel Metering Systems: Carburetors and Fuel Injection

Engine Fuel Metering Systems: Carburetors and Fuel Injection

Fuel metering systems control the ratio of fuel to air entering the engine's cylinders. Getting that ratio right — across all power settings, altitudes, and temperatures — is essential for efficient combustion, engine longevity, and flight safety. Two systems dominate general aviation reciprocating engines: the float-type carburetor and the continuous-flow fuel injection system.

Float-Type Carburetor: How It Works

The float-type carburetor meters fuel by balancing atmospheric pressure against engine intake vacuum. A hollow float rides on fuel in a chamber; as fuel level drops, the float falls and opens a needle valve, allowing fuel in from the tank. When the level rises, the float closes the valve. This keeps fuel at a nearly constant level relative to the main discharge nozzle.

Air entering the engine passes through a venturi — a narrowed passage — where velocity increases and pressure drops. That low pressure draws fuel from the discharge nozzle in proportion to airflow. The throttle valve (butterfly) downstream of the venturi controls the volume of the air-fuel charge entering the intake.

Main metering circuit supplies fuel at cruise and high-power settings. A fixed main metering jet limits maximum fuel flow.

Idle circuit takes over at low throttle openings, where venturi velocity is too low to draw fuel from the main nozzle. A separate idle jet and idle air bleed provide the correct mixture when the throttle is nearly closed.

Accelerating pump circuit prevents the momentary lean condition that occurs when the throttle is opened rapidly. A spring-loaded pump forces an extra shot of fuel into the airstream, smoothing acceleration.

Mixture control adjusts the fuel-to-air ratio as altitude changes. As an aircraft climbs, air density decreases; without adjustment, the mixture becomes progressively richer. Moving the mixture control toward "lean" restricts fuel flow through the main metering system, restoring the correct ratio. Moving it to "idle cutoff" shuts off fuel entirely — the correct way to stop an engine.

Float circuit maintains the correct fuel level in the bowl, as described above.

Carburetor Ice

Carburetor ice is a serious hazard. As fuel vaporizes in the venturi, it absorbs heat, and the venturi's pressure drop further cools the incoming air. Ice can form even at outside air temperatures as high as 70 °F (21 °C) with high humidity. Ice builds on the throttle valve and venturi, restricting airflow and causing a gradual loss of power — often without an obvious cause. Carb heat, which routes warm air from around the exhaust to the carburetor inlet, melts the ice. A temporary drop in RPM after applying carb heat is normal and confirms ice was present.

Continuous-Flow Fuel Injection

Fuel injection systems replace the carburetor with a fuel servo that meters fuel based on airflow and throttle position, then continuously deliver fuel near each cylinder's intake valve. This gives more uniform mixture distribution, better throttle response, and eliminates carburetor icing as a concern (though induction icing at the air inlet remains possible).

Fuel travels from the aircraft fuel system into the fuel servo (also called the fuel control unit). The servo measures incoming airflow using an air metering force against a diaphragm and simultaneously senses fuel pressure. It produces a metered fuel pressure proportional to airflow. The mixture control on an injected engine works similarly to a carburetor — it adjusts the servo's fuel flow to compensate for altitude-related air density changes.

From the servo, metered fuel passes to the flow divider (sometimes called a fuel distributor valve). The flow divider ensures that fuel reaches all cylinders simultaneously and at equal pressure. It also houses a check valve that holds residual fuel pressure when the engine is shut down, helping prevent vapor lock on hot restarts.

From the flow divider, individual fuel lines run to injector nozzles mounted in each cylinder's intake port. Each nozzle incorporates a calibrated orifice. Nozzles must be matched (within manufacturer tolerance) so that each cylinder receives an equal share of fuel. A clogged or partially restricted nozzle causes a lean condition on that cylinder — a common cause of rough running or an abnormal exhaust gas temperature (EGT) reading on that jug.

Common Malfunctions and Symptoms

| Symptom | Likely Cause | Corrective Direction |

|---|---|---|

| Rich idle, black smoke at low power | Idle mixture set too rich or idle jet clogged with debris | Adjust idle mixture; inspect idle circuit |

| Lean mixture at altitude | Mixture control not leaned; altitude compensator malfunction | Lean per POH; inspect servo |

| Rough running, one cylinder lean | Clogged injector nozzle | Remove, clean, or replace nozzle per MM |

| Gradual RPM loss (carb-equipped) | Carburetor ice | Apply carb heat; inspect heat system |

Safety in the Shop

Avgas (100LL) is highly flammable. Wear fuel-resistant gloves when handling any component wetted with fuel. Ensure the work area is well-ventilated and free of ignition sources — no open flames, no sparks from tools, and electrical equipment must be rated for the environment. Have a CO2 or dry-chemical fire extinguisher within reach. Before disassembly, verify the fuel supply is shut off at the selector and that lines are drained into an approved container. Ground the aircraft or training unit and the container before draining to prevent static ignition. Follow your shop's hazardous materials disposal procedures for recovered fuel. When returning a fuel system component to service after maintenance, always perform a fuel system leak check per the manufacturer maintenance manual before engine run.