Captain Obvious

Actually I didn't do anything quite so glamorous. I've got my electronic idle air control valve wired "backwards" and I'm using the stock BCDD cut control signal to open it once the car starts moving. So when the car is below 10 MPH and the BCDD disable signal is high, my idle air valve closes. And when the car is above 10 MPH, the idle air valve opens up a little to reduce manifold vacuum when coasting. Now because of the way I'm doing it and the simplicity of the way I'm using the existing circuitry, I didn't have a lot of adjustability. I have the idle air valve "almost as closed as I could get it" when powered, and that also dictates how much the valve will open when it IS unpowered. So I get an effective 3-400 RPM boost off idle once the car is moving. At idle, I add the leakage past the idle air valve plus the air bypassing the idle speed control screw, and I get my total idle air. If I coast to a stop in neutral, I can see the switch close and the idle air valve close and the RPM's will drop from 1200 down to 700-ish where I want my idle. It's not glamorous and it's relatively unadjustable, but it was simple.

I get that!!

That sounds like what I did with my throttle body work. I switched over to a TB from a Sentra and it has a built in idle air controller. I'm not using it to actively control the idle speed, but I am opening it up once I'm off idle to act like the BCDD. I don't think it does as effective of a job of limiting the manifold vacuum as the original BCDD did, but it's better than nothing.

Great. So you said this device replaces the BCDD. Does it mimic the same kind of behavior? Limits the intake manifold vacuum from going too high?

I doubt the difference in length really matters. I just wasn't sure if you were going for a complete stock result or not. From a functional standpoint, I think you'll be fine. I haven't even really thought about until this thread came up. So the good news is my diff isn't clunking, but I haven't looked at it since I put it in. Maybe checked it once to make sure everything looked OK, and then completely forgot about it. So... I'm guessing it's fine?

Yeah, sorry. For some reason I thought the modifications you made under the throttle body took that into account. I've just been so awestruck with your productiveness and the scale of the prolific improvements that you've been making. I must have been blinded by the bling somewhere else. On edit... You said "I'll remove the plate I made & plug the front side port." Make sure you plug the correct side. If I understand your idle air controller plumbing*, you want to plug the high vacuum side of the throttle plate and use the low vacuum side for your idle air supply. So I'm not sure what you meant by "front side port", but I'm just bringing it up to make sure you think about it. * (which is not guaranteed)

You can get away with that on the ZX because of the different BCDD design, but on the 280Z, you have to block off at least one of the internal holes.

Sorry, I wasn't paying enough attention. On the 280, you can't just slap a plate on the bottom to block off the BCDD. If you do that, you'll have a huge passageway around your throttle plate:

All the pics are dead, but here's a thread I put together a bunch of years ago about my diff strap project: https://www.classiczcars.com/forums/topic/55955-my-new-diff-mount-and-strap-project/ And here's a couple of the pics that don't show up in that thread anymore. Here's the older and newer side by side: I found early strap to be 537mm and the later strap to be a little shorter at 529: And here's what I ended up making for my car. I used heavy duty nylon web strap and made my own:

Right. When you blip it, fuel pressure should spike up. But revving and holding probably won't raise the pressure much. Maybe a little, but not much. And as for holding pressure on the system.... There are two things. First, on the supply side, there should be a check valve screwed into the fuel pump the hold pressure towards the rail. And second, on the return side, the fuel pressure regulator should hold pressure in the rail. The exact pressure number on shut-down is a little fuzzy because of the timing of events, but it should be "non-zero" and probably "between 15 psi and 25 psi". We could get into the reason why it's hard to predict on engine shut-down, but it's academic and probably not necessary for what you're working on.

But with no load on the engine, it really doesn't (shouldn't) take a lot of throttle to get the RPM's way up sitting in neutral. Point is... Sitting in neutral there won't be a lot of change of manifold vacuum at any steady state RPM level. And because of that, steady state fuel pressure should be about the same. You should see a small increase in pressure as you raise the engine speed, but not much. You'll see a blip nearly to 36 psi when you goose it, but steady state should be about the same regardless of the engine speed assuming that speed is constant. Does that make sense?

It does respond that fast. You should see the pressure blip up when you goose the throttle (because there is a quick blip reduction of manifold vacuum).

Fuel pressure should be 36 psi minus manifold vacuum (in psi). So for example, if manifold vacuum is 16 inches of Hg, then the fuel pressure should be (36 - 7.8) or about 28 psi. The conversion for inches of Hg to psi is about half. One inch of mercury is about a half psi.

The symptoms of breaking up worse under load do make me think of an ignition related issue. Your different colors on the plugs might be a tuning issue as well, but load dependent misfires would have me looking at ignition stuff first. Plug wires arcing to ground somewhere, or coil wire arcing over to someplace it should not be going. Have you put in new cap/rotor/wires?

Great stuff! Funny though, (in my immature 13 year old brain) this me chuckle :

Not sure it really matters, but I think the 55425-N4300 is the strap for the 280's with the R200 diff. if you've got a 240, then I believe you want the 55425-E4100. My measurements indicate that the two straps are different lengths.

Yeah, I'm not saying that's the problem, just tossing out ideas and possibilities. From the other stuff you mentioned, like differences between plug groups, it certainly sounds like it could be an issue elsewhere. Happy hunting, and hope you get it running the way you want it!

Looks great. So when are you going into production? It sounds like there are a couple other owners who want those. Aint that the truth!!

Exactly. The only way the port into the side neck of the canister gets down into the carbon part of the can is through the little hole you plugged with a screw. I suspect that many of the original purge valves built into the cans don't work correctly anymore and I think your use of a new aftermarket purge valve is a cool innovative way to get around that. You just missed the mark a little bit with your original implementation. Glad to help.

I think I did that already. That's what my first three posts were. My first post was pointing out that your modification does not work. My second post was telling you how to fix it and actually make it work. My third post was additional info for you to help you understand why what you have done does not work.

That video was done by someone who knows how the system works. I'm not asking if he knows what the purpose of the purge valve is. I'm asking YOU what the purpose of the purge valve is. What is the purpose and why would you ever want it to open?

Yeah, if it's as simple as it sounds, it's likely that your wiring change is not what's causing the problem. However... Note the word "directly" that I used earlier. The reason I said that is because there may still be a problem with the 72 distributor you threw in there which is causing a problem. Points not set correctly. Dwell issue. Spark scatter from bushing wobble. Leaky condenser killing spark at higher RPM... Lots of problems could be caused by the distributor but might not be caused by the wiring modification you made. Point is, I'm not yet giving your 72 distributor a thumbs up, but I don't think the problem is your wiring change. Does that make sense?

I was actually talking about either the stock purge valve, or the aftermarket one you put in. Doesn't matter. Why did Datsun put that valve there? What is it's function? Why would you want to have that valve open? Ever? Why didn't you just cap off all the lines at the engine and pull the carbon canister off your car? I'm trying to confirm that you know what the purpose of that purge valve is.

OK, then let's start simple.... Why would you want to have the purge valve open? What would be the reason to ever do that? Explain to me why having the purge valve open is a good thing.

Adding that extra valve the way you have done it does not "work like stock". It does nothing functional at all with respect to vapor mitigation, and in fact, could actually cause engine damage. The stock system does this... The fuel vapors that come off the tank and temporarily stores them in the carbon canister. Then under certain circumstances while the engine is running, those vapors are pulled back out of the carbon canister and disposed of by burning them in the engine. That's what the stock system does. The system, as you have modified it does this... The fuel vapors that come off the tank are stored in the carbon canister. Until the carbon in the can becomes fully saturated and cannot hold any more vapors. After that point, any vapors that are pushed into the tank are simply vented to atmosphere through the fiberglass filter on the underside of the can. Also under certain circumstances while the engine is running, the purge valve will be opened and an unknown and unpredictable amount of fresh air will be pulled into the engine leaning out the fuel mixture an unknown and unpredictable amount. That's what you have now.