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BCDD Adjustment and Bench Testing

Subject: BCDD Adjustment and Bench Testing
by NoferTrunions on 2022/4/23 4:59:16

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.