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BCDD Adjustment and Bench Testing
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I'm dealing with a L18 DCH340 which is running lean when in the primary before the secondary opens. I'm trying to independently test and confirm the BCDD is functional - of course it has to be adjusted if necessary. (I don't want to be dealing with a malfunctioning BCDD when sorting this lean issue.)

Adjustment of the BCDD appears to do nothing. (I'm never had a BCDD behave this way.)

The solenoid is working.

I've read the Tech Wiki on it and under the Adjustment section, I simply do not understand Steps 1 and 2:

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.


QUESTIONS about 1):
"level set at idling" - the vacuum level at idle cannot be set. I don't understand.
"gradually decreases" - I would never use the word "gradually", "quickly" would be more appropriate. What am I missing?

QUESTION about 2):
"Check the BCDD operating pressure" - no instructions are given how to do that.

What I remember from like 8 years ago (and I have a terrible memory), is that the BCDD introduces an "Idle Hang" when you abruptly lift on the pedal. Adjusting the BCDD will lengthen or shorten the duration of this "hang."

I also noticed back then when the BCDD is "operating" there is a very loud hissing that is unmistakable coming from the carburetor. If the BCDD was incorrectly set and it was hissing all the time meant it was "stuck" at the "hang" state even when idling.

Anyhow, I am chasing gremlins and have a new Tomco 5321D kit to change everything in the carb and also a new choke pulloff to replace the dead one, but I'd like to bench test my BCDD's (I have 2 spares) to confirm the one I put on the carb is good.

Posted on: 4/23 4:59
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Re: BCDD Adjustment and Bench Testing
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BCDD was used on L-series engines to temporarily "hang" the throttle in order to reduce emissions. The A-series engine instead used the Throttle Opener

Quote:
"level set at idling" - the vacuum level at idle cannot be set. I don't understand.
Yes, you are correct.

"decreases to the level set at idling" should be "decreases to the level seen at idling". The BCDD temporarily causes the manifold to stay open, thus the vacuum will be lower than normal

Quote:
"gradually decreases" - I would never use the word "gradually", "quickly" would be more appropriate. What am I missing?
To me it seemed gradual, as in it took its sweet time. But to others maybe it seemed quick

I was used to the throttle coming back to idle as soon remove the foot. But with BCCD it took a bit of time. This is how most 2000s vehicles work although it varies a lot by the particular model, some seems to do it quickly, others take notoriously longer such as the 2018 WRX


Quote:
"Check the BCDD operating pressure" - no instructions are given how to do that
this was from the Datsun factory service manual. i think Nissan meant vacuum when they said pressure (in other words vacuum pressure)

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

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

To measure this "pressure" (vacuum level), use a vacuum gauge

Posted on: 4/23 21:05
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Re: BCDD Adjustment and Bench Testing
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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


Quote:
= BOOST CONTROLLED DECELERATION DEVICE (B.C.D.D.) =

== DESCRIPTION ==
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.

Open in new window

Figure EC-23 Boost controlled deceleration device


== INSPECTION ==

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

Open in new window

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


== 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

Open in new window

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

Open in new window

Figure EC-28 Adjusting control valve

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Figure EC-29 Adjusting setscrew

Notes:
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


== REPLACEMENT ==

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.

Attach file:



png  Figure EC-23 Boost controlled deceleration device.png (55.16 KB)
174_6264869e51593.png 979X559 px

png  Figure EC-24 Connecting vacuum gauge.png (68.02 KB)
174_626486c7bf27b.png 624X500 px

png  Figure EC-25 Characteristic curve of B.C.D.D..png (10.81 KB)
174_626486d509b0a.png 512X576 px

png  Figure EC-26 Characteristic curve (high operating pressure).png (14.85 KB)
174_6264873e5c6fd.png 532X992 px

png  Figure EC-27 Characteristic curve (low operating pressure).png (13.92 KB)
174_626487473accb.png 1104X464 px

png  Figure EC-28 Adjusting control valve.png (11.90 KB)
174_6264874df3bba.png 500X500 px

png  Figure EC-29 Adjusting setscrew.png (16.38 KB)
174_62648753e0216.png 560X464 px

png  Figure EC-30 Removing B.C.D.D..png (13.01 KB)
174_6264875b91b69.png 528X464 px

Posted on: 4/24 0:16
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Re: BCDD Adjustment and Bench Testing
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So maybe I was wrong about how BCDD works. It looks like instead of opening the throttle, it changes the mixture for a time. From the description it suddenly lets in air (which would raise the idle speed) and then "decrease gradually" and finally returns to normal

"B.C.D.D. operates to supply additional mixture gas of optimum mixture ratio" -- by that i figure it leans out the mixture, but who knows?

The FSM uses the phrase "decrease gradually" to an intermediate vacuum level before going to normal


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

As for bench testing first I would test the diaphragm. Is it leaking? Maybe you could put your finger over the air passage, then use a vacuum gun to pull vacuum and if it will hold

Which kind do you have?

BCDD without Control Valve
Open in new window


BCDD with Vacuum Control Solenoid Valve
Open in new window

Posted on: 4/24 3:01
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Re: BCDD Adjustment and Bench Testing
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ddgonzal: Thank you so much for the BCDD pages from the engine manual

My DCH-340-12 has the solenoid.

Mine was working last time I rebuilt the carb, now it is not working - it does nothing.

I seem to recall the adjusting screw was similar to the idle screw: turning it CW resulted in more hang. ALSO gently pushing the adjusting screw is equivalent to turning it in (CW) - you can feel the servo action on the diaphragm. Currently nothing happens no matter how I adjust screw.

It is my understanding that when the solenoid is energized, the BCDD is turned off.

There are articles about how to keep solenoid energies to completely disable the BCDD.

To disable, why not remove the solenoid plunger which appears to be a needle valve that is opened when energized

Last time I rebuilt carb, I wanted to disassemble the BCDD to check the diaphgarm (is there just one? cutaway drawing is too complicated) but I aborted because I was afraid that I would tear the diaphragm trying to separate the housing halves - the diaphragm adheres to the sealing surface with great force. If one HAD to remove, perhaps heating with a blow dryer would help separation.

I think I'm going to try to bench test my 2 spare BCDD's before I rebuild the carb.

NOTE: the Tomco 5321D kit is amazing - has 4 air horn gaskets including the one I need that I have NEVER been able to find in a kit. Talked with Tomco and they consolidated kits that varied by only a gasket of two reducing the typical kit inventory of 600 to 200. So the 5321D is the Gold Standard in my opinion. AND the gaskets and needle/seat are alcohol resistant.

Posted on: 4/24 17:07
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Re: BCDD Adjustment and Bench Testing
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yes there are two diaphragms

* one on the body-to-carburetor surface
* one on the body-to-adjuster side

The first doubles as the "gasket" to bolt onto the carburetor, it is actually a vacuum diaphragm that moves the inlet passage ball valve. The other one is between the two housing halves, and i think moves slightly when the adjuster is turned

To remove the BCDD take the screw on the pointed end, and the other two screws (marked '1' in the diagram)
Open in new window


If you remove all the screws the adjustment screw plate can come free. The diaphragms don't always tear, can sometimes be reused. Does the Tomco kit come with new diaphragms?

Posted on: 4/25 0:14
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Re: BCDD Adjustment and Bench Testing
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The Tomco kit has what appears to be a gasket between the BCDD assembly and the carb body.
Clearly NO diaphragms.

Posted on: 5/5 5:44
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