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TIP: Idle Hang, High Idle, BCDD, Carb Hissing


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I think I've finally solved my idle problem:

 

The symptoms were that sometimes I would get my properly set idle and then there would be a idle hang or high idle.

 

I noticed when trying to balance the mixture and idle screw:

  • the carb would make a loud hissing noise, but when the idle got really low, maybe 600 rpm (way lower than spec), the hiss would stop.
  • When I would open the throttle a bit, the hiss would return.

 

Carbs usually hiss when idling, but the hiss usually does not go away.

 

Then, it finally hit me, idle hang is exactly what the BCDD does (but only for a short period of time.)

 

So I turned the BCDD screw CCW (guessing, couldn't remember which way it went) and low and the hissing stopped.

 

I set the carb for idle and drove around for a bit.

 

The Idle hang (AND hissing) occurred again, so I CCW'd the BCDD again until it stopped again.

 

Another test drive and the idle hang is gone.  (If it comes back, more CCW)

 

I think I've finally tuned the BCDD and solved this problem.

 

When the BCDD is inactive (not hissing) the Hitachi DCH340 is very quite at idle - I'd actually say it "doesn't hiss" for all intents and purposes.  (I think bigger engines = bigger hissing.)

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Unexplained high idle could be over sensitive setting of the BCDD, or carb secondary is part open, stuck or something jammed in it..

 

Did you check the BCDD???? The BCDD is like a carb inside the carb. When activated it allows gas and air into the intake regardless if the throttle plate is closed or not.

 

High and lower random idle can also be the BCDD set to sensitive. Turn adjustment counter clockwise.

 

The closest you came to adjusting the BCDD was this......

 

I did the disable BCDD by hotwiring its solenoid - I found that one on another Datsun site (datsun1200 maybe) but it made no difference when the idle was too high.

 

I'm not sure but if you blip the throttle, if the BCDD is adjusted properly, won't it cause an idle hang of a second or two and then rpm comes down to idle speed. If the BCDD is not working, when you blip the throttle, there is no hang and rpm drops smoothly back to idle speed.

 

If you had adjusted the BCDD back in December you wouldn't be posting today that you solved the problem.

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There were two things I was not aware of and found helpful:

  1. You have to repeatedly adjust the BCDD until you get it sufficiently backed off.  Doing it once will not be enough. 
  2. Once you learn the BCDD hissing sound, you can simply listen to the carb and know if the BCDD is operating - this was the real breakthrough that let me nail it.
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There were two things I was not aware of and found helpful:

  1. You have to repeatedly adjust the BCDD until you get it sufficiently backed off.  Doing it once will not be enough. 
  2. Once you learn the BCDD hissing sound, you can simply listen to the carb and know if the BCDD is operating - this was the real breakthrough that let me nail it.

 

 

Five things you weren't aware of. Three were my posts to adjust the BCDD four months ago.

 

1/ not true. Yours may have needed multiple adjustments. I adjusted mine once about half a turn counter clockwise. They usually do not ever need adjusting and if they do it's a small amount, unless the carb was rebuilt and someone fiddled with it. If the operating pressure is at or below the intake vacuum level the BCDD will be in a closed loop and will not shut off. Turning counter clockwise raises the trigger threshold. 1/8 of a turn changes the setting by about 0.8 inches of mercury. Half a turn probably disabled it totally, but all I wanted was my idle back.  If too low, the intake vacuum will hold the BCDD on continuously. 

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My carb could be messed up - it was a basket case but had a stamped tag saying "74 MT" so I figured it was correct.  I checked all the jets and they were correct.  I assembled it according to the parts manual diagram hopefully getting every little spring and slug right.

 

There were instances where the engine was idling "correctly" and at a stop light, the idle would actually increase and then hang.  Once it did this, it was in permanent "high idle" mode.  This was after I checked the BCDD with large blips of the throttle, so I thought the high idle was a vacuum leak.  I spent a lot of time trying to hunt that down to find only some leakage at the throttle shafts.  It also had a worn primary throttle plate that kept the throttle from closing completely onto the idle screw - so that was clouding the issue too.

 

For reference, I don't think I CCW'd the BCDD more than 1/4 turn. 

 

I've finally gotten this monkey off my back and am really enjoying driving it more.  I just ordered a L20B radiator for it from Advance - cool parts guy fixed me up with a $118 matched price when I commented our garage price was $118 a month ago - there was a price increase and it was now $140 (!).  Also found out cool parts dude drives a 4-dr CTS-V Cadi - wow!  (the 2016 is over 600hp now.)

 

So if it idles, doesn't loose water, and continues to not vibrate after hanger/alternator fix, this will close a chapter on this "reassembly."

 

EDIT UPDATE:

 

Putting more miles on truck and I've noticed the BCDD has begun doing its job when I back off.

 

It's possible I may have to CCW it a bit if keeps "drifting."

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