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Boost Controlled Deceleration Device (B.C.D.D) serves to reduce HC emission emitted from the engine during coasting. It is used by some Datsuns, but not by Datsun 1200.
Overview
BCDD starting June 1971 for Datsun 510 USA
B.C.D.D. with Vacuum Control Solenoid Valve (e.g. 1974 Datsun 620 USA with L18 engine)
Boost controlled deceleration device (B.C.D.D) serves to reduce HC emission emitted from the engine during coasting.
The high manifold vacuum during coasting prevents the mixture from complete combustion because of the reduced amount of mixture per cylinder per rotation of engine, with a result that large amount of HC emission is emitted into the atmosphere.
B.C.D.D. has been designed based on the idea of eliminatin this objection. It operates in such a way that, when manifold vacuum exceeds a predetermined value, it provides and additional mixture of optimum mixture ratio and quantity iinto the manifold by opening the separate mixture passage in the carburetor. Complete combustion of fuel is assisted by this additional mixture, and remarkably reduces the amount of HC contained in exhaust gases.
During the period from coasting to idling, the transmission produces a signal which in turn energizes the vacuum control solenoid. As this takes place, the valve is lifted off its seat, releasing the vacuum chamber to the atmosphere. The mixture control valve will then be close, returning engine speed to the predetermined idling.
Others
Not to be confused with "Coasting Air Solenoid Valve" (triangle base on Hitachi DAF328)
Not to be confused with Fuel-Cut Valve or Anti-diesel Valve:
