Diaphragm and piston operation

The carburetor is a variable-venturi type, whose venturi cross section area is increased or decreased automatically by the piston valve CD which moves according to the negative pressure present on the downstream side of the venturi ®. Negative pressure is admitted into the diaphragm chamber ® through two orifices ® provided in the piston valve CD.

Rising negative pressure overcomes the spring © force, causing the piston valve CD to rise to increase the said area and thus prevent the air velocity from increasing. Thus, air velocity in the venturi passage is kept relatively constant for improved fuel atomization and for securing optimum ratio of fuel/air mixture.

... _ AIR FLOW

< ====i NEGATIVE PRESSURE

.... -AIR FLOW

< ====i NEGATIVE PRESSURE