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 - not an aircraft technician straining to understand what went wrong with his carbureted engine.
A quick glance at a Marvel Schebler Carburetor might lead some to believe that these units are far too simplistic for today's modern aircraft. The obvious advantages associated with fuel injection (even fuel/air distribution, no carb icing concerns, etc.) seem to overshadow the value of the much-maligned carburetor. However, a closer, more scrutinizing look reveals an intricate relationship between various sub-systems within these carburetors. The details that comprise these systems vary from model to model. It is only when the functions of these systems are fully understood that one gains a renewed appreciation for their elegantly simple designs. These units have continued to serve us well through the passing of time.
Principles of operation
In the most simple of terms, an aircraft carburetor is a device for introducing and mixing a metered amount of fuel to the cylinders. Unlike direct injection that provides a precise and uniform delivery of the charge directly into the intake port of each cylinder, the atomized fuel from the carburetor seeks the path of least resistance as it travels through the induction tubes. Fuel distribution is far from exact as indicated by split CHT and EGT readings. According to Lycoming, a variance of 150 degrees is not at all unusual. The job of the carburetor is to perform two very basic functions:
1. It measures out an appropriate amount of incoming air
2. It mixes that air with fuel to assure that a proper charge enters the cylinders under all operating conditions.
Fuel from the wing or header tanks is fed either by gravity or by a low-pressure pump to the inlet of the carburetor. The actual pressure available from a gravity feed system is about one PSI for each forty inches of head of fuel (as measured from the surface of the fuel in the tank to the point of discharge into the carburetor). Some carbureted engines are installed on low-wing aircraft or are "Turbo-normalized" and require as much as six to nine PSI of pressure to perform at altitude. These installations would require a gravity feed of some 240 inches of head pressure, rendering the gravity feed method impractical.
As fuel rises in the float bowl chamber, it lifts a float that is hinged to the throttle body. The float fulcrum lever carries a needle valve, the point of which extends into a seat at the inlet of the carburetor. When the fuel level rises far enough in the bowl, the needle valve begins to partially restrict or close off fuel flow. The height of the fuel in the discharge nozzle is controlled by the position of the float and the needle valve in the float chamber. As fuel is discharged from the carburetor, the float lowers and allows more fuel to fill the bowl. In this manner, optimum fuel level is always maintained so long as the float level has been properly set at overhaul.
A partial vacuum created by the piston during the intake stroke draws air through the carburetor. The air passages in both the carburetor and the manifold are designed to admit a sufficient amount of air to fill the cylinders by the end of the intake stroke. The throttle plates' function is to regulate the admission of air to the cylinders, thereby controlling the power output of the engine.