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Re: BCDD Adjustment and Bench Testing

Subject: Re: BCDD Adjustment and Bench Testing
by ddgonzal on 2022/4/24 0:16:37

Below are the instructions from the 180B L18 factory service manual. Sorry I don't have the USA manual anymore, hopefully it is similar

I suspect that the procedure was translated from another language. Not sure what is meant by "idle boost pulse", maybe some Aussie can tell us


The B.C.D.D. serves to reduce HC emissions during coasting.

The high manifold vacuum during coasting prevents the complete combustion of the mixture gas due to the reduced amount of mixture gas available.

As a result, an excess amount of HC is emitted into the atmosphere.

When manifold vacuum exceeds the set value, this B.C.D.D. operates to supply additional mixture gas of optimum mixture ratio.

Complete combustion of fuel is assisted by this additional mixture, and HC emissions are thereby reduced.

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Figure EC-23 Boost controlled deceleration device


1. Connect vacuum gauge to intake manifold

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Figure EC-24 Connecting vacuum gauge

Note: A service plug is installed on intake manifold. Remove this plug and install an attachment which is equipped with vacuum gauge

2. Start engine and observe vacuum gauge while flooring and releasing accelerator pedal abruptly.

Note: This operation simulates the engine coasting condition

3. If B.C.D.D. is in good condition, vacuum gauge will indicate the following phenomena

a. It will suddenly rise up to about 600mmHg

b. It will decrease gradually to 560mmHg and stay there for a while

c. It will drop to idle boost pulse

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Figure EC-25 Characteristic curve of B.C.D.D.

4. If it does not react as described above, set pressure must be adjusted. In such a case, refer to Adjustment


<< High operating pressure >>
When operating pressure is too high, B.C.D.D. remains inoperative and negative pressure decreases with no sustained plateaus while it is falling. See diagram (A). In such a case, adjust operating pressure as follow:

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Figure EC-26 Characteristic curve (high operating pressure)

<< Low operating pressure >>
When operating pressure is too low, engine rpm will not fall to idle speed

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Figure EC-27 Characteristic curve (low operating pressure)

1. Remove rubber cap at bottom of B.C.D.D.

2. Turn adusting screw slightly. Then, race engine and confirm that operating pressure is at the specified value

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Figure EC-28 Adjusting control valve

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Figure EC-29 Adjusting setscrew

a. Turning adjusting screw one quarter rotation will cause a change in operation pressure of about 20mmHg
b. Do not fit tip of screwdriver tightly into screw slot

3. After adjustment has been completed, install rubber hose and service plug in their original position


1. Remove three B.C.D.D. securing screws, B.C.D.D. assembly can then be taken out from carburetor

Note: There are six screws at the bottom of B.C.D.D. Three of them are B.C.D.D. securing screws and others are B.C.D.D. assembly screws

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Figure EC-30 Removing B.C.D.D.