Weird hanging RPM issue

[AZDatsun]

My 78 would hang while driving. Ended up adjusting all the throttle linkages to remove the slop and putting a new return spring on it and that solved my issue. When the idle would hang I could move the linkage a tiny bit in the engine bay and it would drop the idle back down


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[Captain Obvious]

12 hours ago, Pochie45566 said:

When I start the car the idle is at normal, but once I press the gas pedal the rpm's will not come back down, they will stay up until I turn off the car at a stoplight or something.

I guess it's physically possible that something inside the BCDD is sticky causing it to "latch" into a semi-open position and staying that way until the engine is shut off. But I'm considering that very unlikely.

We can talk about BCDD diagnostics, but that will have to be after you pull up on the throttle linkage. Make sure your dashpot isn't sticking and pull up on the throttle linkage.

Here's a pic showing the linkage and the dashpot. To increase the engine speed, you push down on the linkage here. So to return to idle, you pull up:

[Pochie45566]

Okay, I so was pretty sure that my BCDD was gone so I ordered a block off plate, used silicone gasket maker, and installed the block off plate. When I start it now, the idle immediately goes to 1750 rpm and cannot be brought back down. the AFR is reading 11.5 so Im pretty sure its not a vacuum leak, that and it would take a pretty obvious vacuum, leak to raise it 1000 rpm. Am I supposed to block off any vacuum lines for the BCDD? What's going on? It has to be getting extra air from somewhere!

[siteunseen]

There's a hole you have to fill with JB Weld when you block of the BCDD I'm pretty sure. Those things don't go bad, they just need fine tuning. 3 or 4 laps around the block and I had mine dialed in.

Here's some info.

https://www.google.com/search?client=ms-android-americamovil-us-revc&source=android-home&source=hp&ei=SV1-X-6qGoiQ5wKqg7vYBQ&q=bcdd+block+off+classiczcars.com+&oq=bcdd+block+off+classiczcars.com+&gs_lcp=ChFtb2JpbGUtZ3dzLXdpei1ocBADOggIKRAWEAoQHjoCCCk6BQgpELEDOgcIKRDJAxBDOgUIKRDJAzoFCCkQoAE6BQgpEM0COgQIKRBDOgsILhCxAxCDARCTAjoFCAAQsQM6CAgAELEDEIMBOgIIADoICAAQsQMQyQM6BQgAEMkDOgQIABAKOgcIABDJAxANOgQIABANOgUIIRCgAToFCAAQzQI6BQghEKsCUKsXWKBsYJN5aABwAHgAgAHEAYgB2RSSAQQwLjE5mAEAoAEBsAEP&sclient=mobile-gws-wiz-hp

Edited October 8, 2020 by siteunseen

[siteunseen]

Here's something about the bcdd that might be useful.

 

[Yarb]

Here is a post that Captain Obvious contributed that fixed my same issue. I plugged both holes although only one is necessary with tapered rubber plugs from Ace hardware. Covered each with high temp silicone. These can be removed if necessary if needed. Here’s the diagram. Thanks CO!!

[Captain Obvious]

Yarb, My pleasure.

Pochie45566, so as mentioned above... You can't just slap a BCDD blockoff plate on there and call it a day. Doesn't work.

And your A/F ratio is 11.5 at 1750 RPM under no load? I'm no expert on the subject, but that's something that probably needs some attention as well.

[Yarb]

Yes Sir. I notified ZCD with the diagram and notes to pass along/add to the sales post when purchasing the plate to help others. Guess it didn’t make the press!