There's more to the fuel system than black and red knobs.
Like humans, your engine has needs. About 14 parts of air for every part of fuel, mixed and distributed with reasonable care, will, with the help of compression and on-time sparks, create combustion, glorious horsepower, and carriage for the $100 hamburger.
Simple needs so they may seem, but fulfilling them can prove more complex in execution. Unlike automobile engines, the typical aircraft powerplant spends little time revving up and down through its power band; instead, we give the horses free reign on takeoff and barely pull on the leather at all during cruise. The magic in tuning carburetors comes in trying to make an engine's transient response crisp and predictable. Modern electronic fuel injection replaces much of the abracadabra with microchips, but car makers spend a tremendous amount of effort in fine- tuning a model's "carburetion."
Because aircraft requirements are more specific and easily met, our airplanes have comparatively rudimentary fuel-delivery systems. We get single-barrel carburetors or continuous-flow, mechanical fuel injection — nothing to make Detroit or Hamamatsu stand up and take notice, but it works for our purposes. Discounting old or uncommon models of aircraft, chances are excellent that you have a Marvel-Schebler carburetor (since known by many other corporate titles but now made by Precision Airmotive.) Or, you might have a fuel injection system made by Bendix (also now owned by Precision) or Teledyne Continental Motors. That's really it — early Bonanzas, Cessna 310s, and Navions among others used pressure carburetors, a hybrid system that most resembles single-point fuel injection still found in some cars. They are rare and getting rarer thanks to dwindling parts supplies.
Carburetors represent the cheapest and simplest method of turning the fuel in the tanks to a combustible mixture. A very basic device, the carburetor includes a tube-like throat and throttle mechanism, which act as an air guide and restrictor, respectively. About midway down, the throat constricts at the venturi, whose presence helps accelerate the airflow, an important consideration given that most carburetors are located below the engine, and gravity will have its way with large, unatomized fuel droplets. Atomization describes the ability of the fuel to remain within the induction airflow as a mist, as opposed to larger droplets. Fuel, carried in a bowl surrounding the lower portion of the venturi, is pulled into the intake-air stream by vacuum through the main nozzle.
Your airplane's throttle directly controls a butterfly valve near the carb's outlet — just below the flange nearest the engine. The mixture control connects to a small rod that restricts fuel flow to the main nozzle's lower end, which is submerged in the float bowl. Naturally, the carburetor will introduce fuel in proportion to the air flowing through the venturi, but the wide variations in the engine's fuel/air ratio needs makes the mixture control necessary.
Many pilots go years without ever thinking about the carburetor. It does yeoman duty, asking only occasionally for some warmth (in the form of carb heat) to keep ice accumulation at bay. Carb ice can form almost any time there's sufficient moisture in the air. Some models are more susceptible to icing than others, so know your airplane and be alert for roughness and/or a drop in manifold pressure or rpm. Always apply carb heat fully and immediately; if icing conditions persist, you can always reduce the amount of heat later, but the first priority is to melt whatever ice has already formed.
Other ways to know when your carburetor is asking for some attention include indications of abnormal performance, like an inability to lean without excessive roughness, unusually high or low fuel consumption, and atypical rpm rise on shutdown. You can quickly test your engine's idle-mixture setup by watching the tach on shutdown; when you pull the mixture back, be looking for about a 50-rpm rise before the engine quits. More of a rise means the idle mixture is too rich, and little or no rise suggests a lean idle mixture; either way, have the setting checked, and take into consideration the temperature and altitude during the test.
Few moving parts reside within the aircraft carburetor, but they are subject to wear, particularly the throttle shaft. A steel shaft supports the throttle butterfly and connects to the panel control through a lever and cable. The carb body is aluminum, so a softer bushing is required to keep the throttle shaft from eating up the carb; these bushings, one per side of the butterfly, can wear to the point of admitting unfiltered air into the venturi or allowing the throttle plate to bind in the throat. You can check the bushing by grabbing the end of the shaft the next time the cowling's off; it should not shift perceptibly side-to-side.
Though strictly not a moving part, the venturi assembly in some Marvel-Schebler carbs has experienced problems. For a time, they came with two-piece cast venturi assemblies; the inner ring was press-fit inside the main venturi. Through field experience, the company discovered that a backfire or carb fire could weaken the legs holding the inner venturi into the carb throat. Mostly the piece would fall out into the air box, but a few escaped through the engine. Unless the failed, loose part blocked or jammed the throttle valve, the carb would continue to function. Precision, owner of the line of carbs, began supplying single-piece venturi castings last year. The Federal Aviation Administration has not yet made retrofit of the single-piece venturi mandatory by an airworthiness directive, but Precision thinks regulatory action is possible. Look for a stamped "V" on the carb's data plate if you think you have the newer single-piece venturi.
Occasionally the induction system downstream of the carburetor can leak, either through broken or worn rubber slip connectors or by way of fractured tubes. Either way, the resulting mixture will be leaner than desired and the engine often will exhibit uneven exhaust-gas temperature spreads from cylinder to cylinder. Such a compromised induction system also admits unfiltered air, which will over time wreak havoc on the top end.
While it's true that carburetors remain the least expensive setup for light aircraft, they are not the most efficient, and are poorly suited for the rigors of turbocharging. Powerplant engineers discovered that decades ago and attempted a solution by introducing direct fuel injection. In this system, fuel is brought to the engine right at the intake port so that, in theory, each cylinder would receive the same amount, no matter its location. In most installations, the induction air path remains somewhat convoluted, and unusual are the fuel-injected engines owning very low inter-cylinder EGT spread. Exceptions are the handful of Continentals with tuned induction systems; the lengths and diameters of the individual tubes are varied so the air flows equally to all cylinders.
There's more in common between the Bendix and TCM systems than there are differences. Each requires a high-pressure engine-driven pump and an equally powerful boost pump, whether the airplane is high wing or low. Fuel from the pumps encounters a metering block attached to the side of a throttle body; this block adjusts the fuel sent to the individual injectors according to throttle- and mixture-control positions. Think of the throttle and mixture levers as spigots, the wider they're open, the more fuel is likely to flow to the engine. Engineers call these systems continuous flow, since the fuel pours into the intake ports regardless of the position of the valve. Automotive systems are timed to squirt fuel in only while the intake valve is open.
The single major operational difference between the Bendix and TCM systems involves the use of the boost pump. Bendix's metering block takes additional information from impact tubes in the intake-air stream. They bias the fuel delivery according to how much air the engine is ingesting; this joins with a device to regulate incoming fuel pressure to make the system relatively insensitive to boost-pump operation. In other words, turning on the pump has no direct influence on fuel flow unless the engine-driven pump has failed. That's why you can turn on the boost pump for takeoff and landing and not worry about the mixture.
That's totally unlike the TCM arrangement, where fuel flow out is directly proportional to fuel pressure in. Pilots must use care in operating the boost pumps, especially in the High positions, since the resulting mixture could turn so rich as to shut down the engine. Obviously, silence is not what one expects upon engaging the boost pump. Continental has used this characteristic to regulate fuel flow on turbocharged and some normally aspirated installations. In the turbo airplanes, fuel flow must be proportional to air flow through the engine, as determined by manifold pressure. An aneroid in the fuel pump meters output according to MP, so the flow will follow the manifold pressure, even if the power controls are untouched. In nonturbo applications, TCM allows the aneroid to reduce full-rich fuel flow with lowering manifold pressure, providing a form of automatic leaning during the climb.
Modern aircraft fuel injection is most sensitive to dirt. Because the injector nozzles include small orifices, even minute particles can block fuel to one or more cylinders. Discovering this condition is simple with a multicylinder EGT system; without it, the blockage has to become quite severe before abnormal performance will tip you off. For a time, the engine makers told mechanics to refrain from removing and cleaning the injectors at the annual, largely on the basis of reports of poor removal and installation procedures. It's generally accepted that the injectors should be ultrasonically cleaned (don't poke anything into the injector's bore) at each annual or when any disparity in fuel pressure, fuel flow, or EGT indications are noted. Lycoming has two-piece nozzles available that remove the hazard of misthreading the injector back into the cylinder.
Proper rigging of the metering circuits should be confirmed at the annual inspection. With your fuel-flow gauge calibrated (either against a reference instrument or through trial-and-error in the field), look for specified fuel flows at takeoff and at cruise power settings. Remember, though, that your flow meter is likely just a pressure gauge, and it can be fooled by nonstandard injector sizes or clogged injectors. Also, be certain your tachometer and manifold pressure gauge reflect something close to reality, or your power settings will be in error.
While peeking under the cowling, check for loose or broken induction tubes and connectors, and fuel stains around the nozzles. Nonturbo airplanes have bleed holes in the injectors to help atomize the fuel, and they will show some fuel staining; this is normal. It is not kosher to see fuel stains around the nozzles on a turbo installation, whose bleed holes are hooked to the induction system to keep them pressurized.
The bottom line is to keep it clean (both the fuel and the induction air) and to quickly seek answers to engine anomalies. If the engine exhibits unusual behavior — unusual rpm rise during shutdown, abnormal fuel consumption, or strange noises or vibrations — do something right away. It could be minor — a slightly blocked injector — or major — like an induction-air leak.
Cleanliness and observation represent the simple needs imposed by the fuel system; no less important is the engine's need of a 14:1 (or thereabouts) air/fuel mixture or the pilot's ache for the $100 hamburger.
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