Lesson 1/Learning Event 2
Most carburetors have five circuits, or passages, for the fuel to flow to the venturi or to other points in the
carburetor bore. These circuits are necessary because different engine operating conditions require different
The five carburetor circuits are the float circuit, the low-speed circuit, the high-speed circuit, the accelerating
pump circuit, and the choke circuit.
The fuel in the carburetor bowl must be kept at a certain level whenever the engine is running. If the level
is too high, too much fuel will be discharged through the nozzle. The engine will use too much fuel, and
large deposits of carbon may form in the cylinders. If the fuel level is too low, the mixture will be too
"lean" to operate the engine properly. The float circuit keeps the fuel at the correct level.
FIGURE 10. FLOAT CIRCUIT.
The float circuit consists of a float hinged at one side of the carburetor bowl and positioned so that it will
operate a needle valve to open and close the fuel inlet. Fuel, under pressure from the fuel pump, enters the
carburetor bowl at the fuel inlet.