10.2 Carburetors, Fuel Injection, and Metering Logic

Carburetors: Venturi, Float, Mixture, and Idle Cutoff

A fuel metering system must deliver the correct fuel mass for the air entering the engine. Too much fuel cools combustion but wastes fuel, fouls plugs, and lowers power; too little fuel raises temperatures and increases detonation risk before misfire. Float-type carburetors meter fuel using the pressure differential created when intake air accelerates through a venturi: the throat is the narrowest point, where velocity is highest and static pressure is lowest, so the low pressure at the main discharge nozzle draws fuel from the float bowl.

A float and needle valve hold a constant fuel level in the bowl (vented near atmospheric) so metering stays calibrated. The mixture control adjusts fuel for air density; pulling it to idle cutoff shuts off fuel at the metering jet so the engine is always stopped with a lean cutoff, clearing fuel from cylinders and preventing run-on.

At closed/idle throttle little air moves through the venturi, so an idle/off-idle circuit supplies fuel from below the throttle plate where manifold suction is high. The accelerating pump adds a momentary fuel shot on rapid throttle opening to prevent a lean stumble, and an economizer (power enrichment) valve adds fuel at high power. , Bendix-Stromberg) instead meter by balancing air-impact and fuel pressures across a regulator and a poppet valve.

Because fuel is delivered under positive pressure rather than drawn by venturi suction, pressure carburetors are far less prone to carburetor icing and tolerate aircraft attitude/maneuvers better than float types.

Carburetor Ice, Carb Heat, and Continuous-Flow Injection

Carburetor icing has three forms per FAA AC 20-113. Fuel-evaporation ice forms where fuel vaporizes and cools the air; it can occur at outside temperatures from roughly +20 F to +80 F (about -4 C to +27 C). Throttle ice forms at or near a partly closed throttle plate where the venturi-plus-throttle pressure drop chills moist air; it is common at low power. Impact ice forms when moist air below freezing strikes screens, scoops, and the heat valve, most rapidly near 25 F.

The venturi can drop air temperature about 70 F (39 K), so ice is possible even on a warm day with high humidity. Ice narrows the throat, so airflow falls and the mixture often enriches because metering does not drop proportionally; a fixed-pitch aircraft loses RPM, a constant-speed aircraft loses manifold pressure.

Carburetor heat routes air through an exhaust heat-stove/shroud to melt ice and prevent formation; because warm air is less dense it lowers power and richens the mixture, and applying heat can briefly worsen roughness as melted water passes through.

Continuous-flow fuel injection (Bendix/Precision RSA series) is a multi-cylinder system that meters fuel by sensing engine airflow. Air mass is measured at the throttle/air-metering body, the fuel regulator sets fuel pressure proportional to airflow, the flow divider (spider) distributes fuel to each cylinder and provides a clean anti-drip idle cutoff, and air-bleed nozzles at each cylinder head spray fuel into the intake port. Fuel flow indication is read at the divider as a pressure proportional to flow.

Compared with carburetors, fuel injection offers less induction icing (no venturi cooling in a fuel-laden throat), better fuel distribution to all cylinders, improved acceleration from positive metering, reduced overheating, and better fuel economy.

A mixture/metering checklist:

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

The shortcut is to ask what changed first - air, fuel, or control position. Ice reduces air; a blocked nozzle starves one cylinder; an intake leak adds unmetered air (lean); a stuck enrichment circuit adds fuel (rich). Leaning toward peak EGT raises EGT until peak, then lowers it past peak as combustion efficiency falls.

Several exam traps recur. First, carburetor heat is hot, unfiltered air, so it richens the mixture and lowers power even when no ice is present - that is normal, not a fault. Second, the mixture enriches with carburetor ice; candidates who answer "leaner" miss that fuel metering does not fall as fast as the airflow restriction. Third, a fixed-pitch aircraft signals ice with falling RPM, while a constant-speed aircraft signals it with falling manifold pressure, because the governor holds RPM constant.

Fourth, an induction air leak and a lean fuel-metering fault both produce a lean cylinder, but only the air leak adds unmeasured air - the distinction drives the repair. Finally, pressure carburetors and injection systems are chosen partly because they largely eliminate the venturi-cooling icing that plagues float carburetors.