Old Faithful

Old Faithful The Marvel(ous) Schebler carburetor By Randy Knuteson October 2000 Everything is simpler than you think and at the same time, more complex than you can imagine. These words were spoken some 200 years ago by a philosopher...


Basic Bernoulli
Two pressures work together to discharge fuel from the carburetor bowl. The atmospheric pressures in the bowl chamber exert a downward force on the fuel within the bowl. And there is also a drop in pressure (a vacuum) at the neck of the discharge nozzle caused by the action of the venturi in the throat of the carburetor. The resulting pressure-differential works to create a push/pull action on the fuel.
Proper fuel metering is accomplished by the strategic placement of the discharge nozzle in the venturi tube. The venturi is situated in the intake airstream at the point of mean velocity immediately upstream of the throttle valve.
Imagine an aircraft wing rolled into a cylinder, dramatically reduced in size, then placed into the bore of a carburetor. In essence, you have formed the aircraft wing into a venturi. The basic law of physics discovered by Bernoulli is as active in the throat of a carburetor as it is across the surface of an aircraft wing. A venturi tube correctly positioned in the throat of a carburetor causes air to move at a much faster rate as it passes through the constriction (See diagram page 80). As air velocity increases, a reduction in static pressure (pressure drop) causes a suction force to draw fuel up the discharge nozzle. The amount of fuel drawn up the discharge nozzle is dependent upon the speed and condition of the air sweeping through the venturi. The greater the velocity, the greater the suction on the fuel in the discharge nozzle.

Unimpeded Airflow
The onrush of air through the throttle body serves to mix and atomize the fuel as it makes its way to the cylinder intakes. This mixing of fuel and air in the throat of the carburetor helps to convert much of the liquid fuel into a gaseous state. Engine speed, efficiency and power are greatly influenced by the quantity and nature of this homogenous charge. Because of this, its extremely important that airflow be unimpeded by sharp bends in the induction or gasket material protruding into the airstream. Such obstacles can produce unsteady or turbulent airflows directly affecting the fuel metered through the discharge nozzle. The quality of the airstream directly influences the metering of fuel. A small piece of garlock material, a loose venturi or any foreign object lodged in the carburetor throat can ruin the metering ability of the carburetor.

Mixture Control
Fuel is always metered in relation to the weight, not the volume of air passing through the carburetor. As the aircraft ascends in altitude it passes through atmosphere that is constantly changing. Pressure, temperature, and density steadily declines. Since thinner air is less dense, each pound of air occupies a greater volume of space. So, as the airplane gains altitude, the volume of air passing through the carburetor will continue to remain proportional to suction in the manifold, but the weight of the air will decrease as ambient air density decreases. Since the air is less dense, a natural richening of the fuel/air ratio occurs. A manual mixture control enables the pilot to alter the ratio of fuel to air. At any given throttle setting, the mixture may require some adjustment to compensate for ever-changing conditions from sea level to altitude. The mixture control also provides a secondary function of completely closing off fuel flow at ICO.

Idle Circuit
The idling system is independent of the main metering jet at low rpms. At idle speeds and up to approximately 1,100 rpm the main jet has little or no fuel passing through it. This is because the throttle valve is almost closed and there is little air swept past the discharge nozzle. At this point, fuel is drawn up the idle bleed tube in the bowl and then through the idle emulsion channel within the throttle casting to ports adjacent to the upper edge of the throttle valve. As the throttle valve is opened, suction at the idle mixture port decreases and the main jet takes over entirely. Fly-holes in line with the mixture port aid in the smooth transition from idle to full power. As the throttle opens it progressively unveils these secondary and tertiary bleed ports. Any sudden opening of the throttle results in a lag between the time the idling circuit stops functioning and the main jet takes over. This is because there isn't sufficient air flowing through the carburetor throat to draw fuel from the main discharge nozzle. An accelerator pump is used to compensate for this delay and eliminate sudden flat spots created by temporary lean mixtures. The accelerator pump is mechanically linked to the throttle and discharges fuel through a tube adjoining the main nozzle. The accelerator tube protrudes into the airflow just inside the primary venturi in the smaller carbs and inside the main venturi in the larger carbs.
Any fuel-borne contamination or corrosion latently within the carburetor can migrate to these idle passages and create a lean condition at low power settings. If the contamination is significant, it will not allow for proper fuel/ air ratios at idle. At sea-level, mixture rise as indicated on the tach should be approximately 25 to 50 rpm. At higher field elevations of 5000-ft. or more, the mixture rise will be more in the neighborhood of 100 to 125 rpm. An idle adjustment screw that requires more than four turns out to achieve a proper mixture rise at ICO is a good indication of contamination in the idle circuit.

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