GENUINE Blur 150, Service Manual

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(A) General theory
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 . 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 through an orifice provided in the piston valve .
Rising negative pressure overcomes the spring force, causing the piston valve to rise into the
diaphragm chamber and prevent the air velocity from increasing. Thus, air velocity in the venturi
passage is kept relatively constant for improved fuel atomization and precise air/fuel mixture.