User Login    
 + Register
  • Main navigation


Lost Password?

Register now!
Fast Search
Slow Search
Google Ad

[Datsun 1200 encyclopedia]


From Datsun 1200 Club

Datsun 1200 encyclopedia | Recent changes | Edit this page | Page history | Switch to MediaWiki mode

Printable version | Disclaimers | Privacy policy
Categories: Fuel System | Emission Control And Tune-Up

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.



BCDD appears to be a kind of idle-air bypass system, letting air past the closed throttle upon deceleration. Thus raising the idle speed, and allowing more complete burning of the fuel. It seems to be an alternative system to the Throttle Opener, a major difference being that BCDD adjusts the air/fuel mixture during operation. BCDD enriches the air/fuel mixture during deceleration. Perhaps it is only meant to work with Air-pump equipped engines? In any case a thorough understanding is not necessary to troubleshoot or adjust the BCDD system.

BCDD starting June 1971 for Datsun 510 USA
DCH340-1dx.jpg DCH340-1ax.jpg
Hose fitting by yellow arrow goes to air cleaner

B.C.D.D. with Vacuum Control Solenoid Valve (e.g. 1974 Datsun 620 USA with L18 engine)

The 1974 620 Service Manual USA states:
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.

Vacuum Control Solenoid Valve

Hitachi DCH340 models with BCDD Control Solenoid, which apparently de-activates BCDD operation.

MANUAL TRANSMISSION: The solenoid is electrically operated according to Speedometer Switch and amp, and a relay box. At speeds below 10mph the valve is operated.
620_speedometer_c.jpg 620_cluster_back.jpg
620 cluster with Amplifier Speed Switch attached to back of speedometer

AUTOMATIC TRANSMISSION: The solenoid is electrically operated according to the Nuetral Safety Switch. When in Nuetral or Park, the valve is operated.


Adjustment requires:

  • Tachometer
  • Precision vacuum gauge

Pull the black rubber cover off the center of the BCDD to expose the adjustment screw.

To properly set the BCDD operating pressure, proceed as follows:

0. Run engine until fully warm. Check and adjust if necessary:

  Idle speed
  Idle mixture

1. Run the engine under no load [in neutral]. Increase engine rpm to 3,000 to 3,500 rpm, then quickly close throttle valve. At this time, the manifold vacuum pressure increase abruptly to -23.62 inHg or above and then gradually decreases to the level set at idling.

2. Check the the BCDD operating pressure is within the specified range.

BCDD set pressure:
-19.69 inHG manual transmission
-18.90 inHg automatic transmission

3. If it is higher than the set level, turn the adjusting screw counterclockwise

4. Race the engine and check again

5. If it is lower than the set level, turn clockwise

6. Race the engine and check again

WHEN THE OPERATING PRESSURE IS extremely LOW, idle speed is not restored. Adjust as indicated above.

WHEN THE OPERATING PRESSURE IS TOO HIGH, the BCDD will not activate at all. Turning the screw clockwise will raise the operating pressure.

Similar Devices

BCDD is 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, which look very similar to the "Vacuum Control Solenoid Valve" but are not located on the BCDD unit.


Retrieved from ""

This page has been accessed 11,874 times. This page was last modified 19:17, 23 April 2022. Content is available under Datsun 1200 Club.