4-10 FUEL SYSTEM
NEGATIVE PRESSURE
DIAPHRAGM AND PISTON OPERATION
The carburetor is a variable-venturi type, whose venturi cross sectional area is increased or decreased
automatically by the piston valve 1. The piston valve moves according to the negative pressure present on
the downstream side of the venturi A. Negative pressure is admitted into the diaphragm chamber 2 through
an orifice 3 provided in the piston valve 1.
Rising negative pressure overcomes the spring 4 force, causing the piston valve 1 to rise into the dia-
phragm chamber and prevent the air velocity from increasing. Thus, air velocity in the venturi passage is kept
relatively constant for improved fuel atomization and the precise air/fuel mixture.
LOWER POSITION OF PISTON VALVE
UPPER POSITION OF PISTON VALVE
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