I have this parts car that i'm trying to get going, to verify some parts and stealing some others.  It starts right up even cold. That was a project.

When cold or warm, I can't get the idle down. I have the TB screw all the way in.  Timing was advanced all the way, so I retarded it some.  Can't find any vacuum leaks except one, two injectors I know are sucking some air.  

I swapped AFM no, change.  Checked AIC and even manually plugged it just to be sure.  Checked all my vacuum lines, brake booster, etc. 

 I just can't believe two injectors could cause that high of an idle?  If anything, I would think that would reduce idle. Thoughts.  

Could a stuck BCDD for some reason cause this?  Reading on this I thought it was off under idle conditions.

I would remove the dashpot, and unplug the bcdd and cold start injector. Then make sure the throttle blade is closing as well as the aar. Hook up a vacuum gauge and see what it's pulling, 17 hg is normal.  Good luck and happy hunting. 8^)

Another thing I thought may be causing your high rpms, the throttle linkage. You could unhook coming out of driver's side firewall on that plastic marble and see what happens. It may be the linkage binding?

Edited January 26, 20178 yr by siteunseen

Cold start valve is off. Also the only linkage is on the throttle body and just the intake. Not connected to pedal at all. CHecked to make sure the TB was closing all the way too. 

Also took off dash pot too as I was thinking that also. 

Edited January 26, 20178 yr by JSM

To remove the BCDD is simply plugging all the vac lines to it?

I don't know anything about 79, but for all the previous years, it's not a simple matter to disable the BCDD.

75-78 has one wire going to the BCDD, and that wire needs to have +12 on it to disable the BCDD. Meaning that if the wire is disconnected, the BCDD will be enabled.

And as for vacuum connections... There are no externally accessible vacuum controls for the BCDD. There are two vacuum ports coming off the throttle body that LOOK like they might have something to do with the BCDD, but they do not. They are both ported vacuum sources that go to small holes in the throttle body bore to control distributor advance and EGR operation. Even though they look like they are for the BCDD, they are not. All the BCDD vacuum ports (both control signals and operation vents) are located internal to the throttle body casting and do not come out to the outside world at all.

Like I said, I don't know 79's details, but if the BCDD is on the underside of the throttle body like 75-78, then I would assume it's pretty much the same.

Yeah it is. My 79 also has the wire to it. I have another TB looks like it is later as it has no wire. I might give that a shot. I also have one of the old Weber 60mm which I don't think has the BCDD on it. My goal was to get the car running perfect stock so when I go to remove all the stuff I don't want I know why. 

And I knew of the ported ported sources as well, but thanks for the reminder. I was curious how folks are "disabling and or removing".  I think that is one reason why some folks are using the 240sx TB. Seeing that on HybridZ. 

Edited January 26, 20178 yr by JSM

Well then I'm not sure I understand your question when you asked "To remove the BCDD is simply plugging all the vac lines to it?"

I thought you were asking if you could remove it from the system (disable it) by plugging the vac lines that go to it. Were you suggesting a way to disable it, or are you suggesting taking it completely off the bottom of the throttle body and replacing it with a solid block-off plate?

Sorry, not trying to come off like a know it all. I know your trying to help. Thank you. It's Bruce right? ^^^^ 

Yes, both actually. From what I've read, it seems most replace TB w/ BCDD  in order to remove.  I understand it's function just not sure simplest way to defeat in order to see if that is what my issue is. 

When siteunseen said unplug I wasn't sure if he was referencing the electric plug or vacuum lines 

I'm assuming if the diaphragm inside is torn unplugging will do nothing. I wasn't sure if plugging the rear vac line would. In the FSM the connection between the TB and the BCDD is internal. 

Edited January 26, 20178 yr by JSM

Major edit, too many words.

Wouldn't disconnecting the power and unscrewing the adjustment screw remove it's ability to change function.  Making it a permanent throttle blade bypass?  Like the idle air bypass,  Readjust the idle screw and you're done.  Turn 6 way out or way in, and 20 to reset RPM.  Maybe.  Seems like the increased idle speed must come from the small passage to chamber 17.  With no solenoid that passage is always open.  I think, I couldn't make sense of the FSM words.

Edited January 26, 20178 yr by Zed Head

Disconnecting the wire will enable the BCDD (which is the wrong direction and not what you want). If you want to completely disable the BCDD, turn the adjustment screw (#6 above) counterclockwise (when looking at the screw head). In fact, you can take that screw completely out.

If the diaphragms inside are intact, the BCDD should be disabled completely. Of course, if one or more of the diaphragms has a hole in it, there's no telling what that thing is gonna do. Could disable, or enable it. Also, if there is a goober of carbon gunk holding one of the BCDD valves open, then things get unpredictable as well.

To test if your problem is the BCDD, you could probably take off the rubber boot on the front of the throttle body and put a small piece of tape over the BCDD holes. Put the boot back on and start the engine. I wouldn't rev the engine for fear of sucking the tape off and down into the engine, but as long as the throttle butterfly stays closed, you would be OK for just a test.

We'll figure this out! 

Zed, the BCDD operation, as I understand it, goes like this...

Chamber #17 is a pneumatic amplifier.
If there is enough vacuum in chamber #17, it will lift plunger #9 off it's seat and allow some vacuum into chamber #18.
And then if there is enough vacuum in chamber #18, it will lift plunger #13 off it's seat and allow air to pass around the throttle butterfly.

So some points of interest...
The further run in the adjustment screw on the bottom, the easier it is to lift #9 off it's seat. That's why the BCDD will operate at a lower vacuum.
When the solenoid is energized (and the pin is retracted) this defeats build up of vacuum in chamber #17. This is because vacuum pulled into chamber #17 from the intake manifold side is just bled off by pulling in air from the upstream side of the throttle body. In order to enable the BCDD, they block the bleed passage on the upstream side of #17 and allow the vacuum from the intake manifold to build up in chamber #17. If that vacuum gets high enough, it will lift the plunger off it's seat.

Keep in mind that I didn't design the thing, but that's my read.