10.2 Carburetors, Fuel Injection, and Metering Logic

Matching Fuel to Air

An engine fuel metering system must deliver the right fuel mass for the air entering the engine. Too much fuel can cool combustion but waste fuel, foul plugs, lower power, and create smoke. Too little fuel can reduce power, raise temperatures, and increase detonation risk before misfire occurs. Carburetors and fuel injection systems solve the same problem in different ways. The FAA Powerplant ACS expects knowledge of the components and the troubleshooting effect of mixture, icing, blockage, and leakage.

Carburetors meter fuel using pressure differences created by airflow. In a float carburetor, fuel level, float valve action, jets, discharge nozzle, throttle position, and mixture control all matter. A leaking float can raise fuel level and create a rich condition. A blocked jet can make the mixture lean. A loose intake connection can admit unmetered air and make the mixture lean. Carburetor heat adds warmer air and usually reduces power because the air is less dense; it may also melt ice and restore airflow.

Carburetor icing deserves careful wording. Ice can form from fuel vaporization and pressure drop in the carburetor even when outside air is above freezing. As ice narrows the throat, airflow decreases and the mixture often becomes richer because fuel metering does not decrease in the same proportion. The engine may lose RPM in a fixed-pitch propeller aircraft or lose manifold pressure in a constant-speed propeller aircraft. Applying carburetor heat may initially make roughness worse as ice melts and water passes through, then improve operation.

Fuel injection systems meter fuel under pressure and distribute it through lines and nozzles. A pressure carburetor, fuel injection servo, flow divider, fuel control, and turbine fuel control are different devices, but the reasoning is similar: fuel flow must match air and power demand. A nozzle blockage may affect one cylinder. A servo impact may affect the whole engine. A leak between a flow divider and nozzle may make one cylinder lean while total fuel flow indication depends on transmitter location.

Mixture troubleshooting uses temperature direction carefully. Leaning from rich toward best power or peak exhaust gas temperature raises EGT until the mixture reaches the peak region. Leaning beyond that can lower EGT because less fuel burns effectively. Cylinder head temperature may rise with lean, high-power operation because combustion is hotter and slower. A rich mixture can lower EGT, cool cylinders, and foul plugs. The correct exam answer depends on the exact starting mixture and symptom.

Use this metering checklist:

1. Determine whether the fault is all cylinders or one cylinder.
2. Compare fuel pressure, fuel flow, RPM or manifold pressure, EGT, and CHT.
3. Inspect for induction leaks, carburetor ice, blocked jets, float problems, contaminated nozzles, and fuel-control rigging.
4. Consider recent maintenance because linkage, mixture travel, and nozzles can be disturbed.
5. Use approved adjustment data because small metering changes can create large temperature and power effects.

The knowledge-test shortcut is to ask what changed first: air, fuel, or control position. Ice mainly reduces air. A blocked nozzle reduces fuel to a cylinder. A leaking intake gasket adds unmetered air. A stuck enrichment circuit adds fuel. Once that first change is clear, the resulting indication usually follows.