Captain Obvious

Haha! Well THERE'S your problem! So about the video of the hunting issue... You're holding you foot steady and it's doing that rocking back and forth between 2500 and 3K all by itself?

Yeah, I agree. It's all wired for it, but they just didn't do it. I guess they just didn't want to add that complexity to the ECU's. Even through the end of the run in 83, they still used the thermotime system instead of having the ECU control the cold start. Certainly a proven brute force system and one less thing for the ECU to have to deal with. Honestly, I've had my cold start valve removed for years and I don't miss it. Sure it takes a couple more cranks on really cold mornings, but for the tradeoffs of being able to completely remove the plumbing leak sources as well as the thermotime switch, I don't really miss it.

Do you still have the EGR system installed? It will use a leaner mix during light cruise (on purpose) and inject some exhaust gas back into the mix. I think that at light cruise, they were trying to optimize fuel consumption and reduce emissions. So they lean it out, recirculate some exhaust gas, bump the timing advance, and also purge the carbon can. All at light cruise. In any event, glad you got over your emissions hurdle and good luck with the project.

I wasn't there when they designed it, but my read on that #21 connection was to allow the ability for the ECU to fire the cold start injector. They never implemented that feature, but it looks to me like it was wired with that ability in mind. Would be a simple matter for the ECU to drag that side of the cold start to ground if they wanted.

Swapping the reluctor wires should not draw anything that would ever pop a 125A breaker. And the wiring between the battery and that part of the car could not support that amperage level without smoking. Something else is going on. Maybe the reversed reluctor was causing the ignition to fire at such an inappropriate wrong time that it was fighting the starter and you were getting locked rotor amperage through your starter? And as for the ballast... I would be surprised to find that the spark is so weak that it is preventing the car from starting. To test that, you could jumper straight across the ballast to get the car to start and then pull the jumper off as soon as it starts? Find out if it will run OK once you get it started? Do it quick so you don't overheat the coil?

So I'm glad you got your car through emissions and are off to the next issue, but... You said your car was running really lean. And you leaned it out even more to get it to pass emissions? Are you sure your readings are all lining up? I don't want you to burn a valve or something because you're running scary lean.

And if you're wondering why those two devices look like they are wired in parallel, it's because they are. But not the way you originally thought. It's confusing because the #21 connection goes to the ECU. but it's essentially a no-connect. The ECU doesn't do anything with it. So with that in mind, you can do this:

Increasing the spring tension will make it run leaner. Goes like this... Flap will open less, ECU thinks there is less air going in, and will supply less fuel accordingly. And I don't remember for sure, but I think I would hope for more than 16 in-Hg at idle. Are you sure you don't have a vacuum leak? Cap off everything not required to make the engine idle and give it another try maybe? For example... Brake booster, EGR system, etc.

That's a beautiful EM-50.

Oh boy. Maybe I'll just stop now. Hey... How bout them Eagles?

Hahaha!!! Of course you can!!

Glad you got it running well. However, just to make my little brain good with the whole ordeal... That same coil ran just fine with the original dual point distributor? Some ideas? Been a long time since it ran at all and anything could have happened between then and now. The coil was borderline before and your moving stuff around and angered the electrons in the coil. Your car runs fine now and it just doesn't matter why changing the coil fixed it.

Datsun actually had some sort of manifold vacuum limit on deceleration even with the early carbs. There was a vacuum actuated diaphragm pull device that would yank on the throttle linkage if the manifold went too high. They kept refining that system through the carb evolution and eventually ported the same concept to the EFI systems. I don't know much about Volvo or Fiat though. I know what they are, but that's about it.

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).