Captain Obvious

And the first word is "Explosive", right. I already know more than I wanted to...

Cool. Glad you're out of the woods. Here's my new tag line. I was talking about carbs at first, but it applies to lots of other things as well! "I've found that surgically clean and actually assembled correctly goes a long way to fixing a lot of issues! Go figure!! "

Yup, I think some (or most) of the high idle issue is due to the BCDD blockoff method. I've never messed with the ZX under the manifold style, but as Zed Head mentioned, you might be able to take off that last other plate that's under there and then maybe use a simple plate to block off the holes. I don't know what (if anything) else that remaining slab does, but you would definitely have to cap the hose connections that go to it if you removed it. That option doesn't exist on the earlier design with the BCDD on the underside of the throttle body. There's no way to ever use just a simple plate because the vacuum ports and valve seats are integral to the throttle body casting itself. You might have a simpler option with the later version you have. Good luck. And here's to hoping solving the problem is as easy as it seems at this point!

It's not that simple. You can't just remove the guts and slap on a simple plate. When you remove the guts, you remove the valve that seals one side to the other. I whipped this up to hopefully illustrate the issue:

New product!! The marketing dept. sees Lightweight Fiberglass Replacement Floor Pans!! You've got the mold, right??

I suspect you didn't block off the BCDD correctly. As siteunseen alluded to above, it's not as simple as removing it and slapping a flat plate in it's place. I've not looked into the details, but I suspect the later ones under the manifold are pretty much the same concept as the earlier ones on the bottom of the throttle body. Do you have any pics of what you did there?

Haha!! What happened? No, wait. Nevermind. Forget I asked... I don't want to know!

Cool, and thanks for the additional pics. Before conclusion... Yes, both of them should have that spring. Someone must have been inside your BCDD in the past, and left that spring out. You probably already know this, but without that spring, it won't work correctly. You didn't get into what kind of problems you were having, but I surmise that without that spring, it would be either unpredictable and erratic, or hang like crazy at high idle the first time you decelerate. Or both? And also just to close up some of the analysis. So if the whole altitude corrector spins with the adjustment screw, then the purpose of the cross dowel is to convey that rotation to the slotted piece that it mates with. And that makes sense since the shaft of the lower valve (the green valve) is threaded. The later model non-CA version essentially ends there with that slotted stub sticking out to be used for adjustment. The rest of the unit is pretty much identical from that point upwards. So if you are interested, basically how this thing works is like this: There is a vacuum chamber above the lowest diaphragm that senses the intake manifold vacuum. If the vacuum in that chamber gets low enough, it sucks the diaphragm upwards and opens the green valve. When the green valve is opened, it allows vacuum into the chamber above it which pulls the upper diaphragm down. Pulling the upper diaphragm down opens the big round nose plunger valve and allows air to bypass around the throttle butterfly plate. Simple, huh? The whole altitude compensator thingie simply automatically adjusts the preload on the lower valve by growing or contracting in length depending on the ambient atmospheric pressure. Again, simple, huh? Haha!! Way too many parts, and be thankful that todays cars do the same thing just by using the computer to open the idle air control valve a little when decelerating. We don't have a computer, and our IACV is a slotted head screw sticking out the top of the throttle body.

I'm not sure there were "partial" CA cars, but there are minor differences between CA cars and others. If you're looking to identify conclusively, I'm sure I can come up with a list of other items to look for other than the catalytic convertor stuff. Stuff like the altitude switch under the dash. I've got two BCDD's here. One is on my original 77 throttle body, and the other is from a non-CA 78 throttle body that I bought as a spare some time ago. I'm reluctant to take my original apart (for fear of damaging one of the diaphragms), but the one from the non-CA 78 is fair game. If I get a chance, I'll snap some shots of that one exploded. Only issue is that since it's non-CA 78, it doesn't have an altitude corrector on the bottom. Just a simple screw on the bottom and the whole thing is an inch shorter than previous years because they simplified the underside. So a question about the altitude corrector... On your BCDD, when you turn the adjustment screw on the underside, does the entire altitude corrector turn with it? Or is the adjustment screw threaded into the underside of the altitude corrector? I'm guessing it's the latter, and that would explain the need for the dowel across the top of the altitude corrector that fits into the slot in the valve. They need that feature to keep the altitude corrector from turning when you adjust the screw underneath.

Glad you found something with the front three cylinders too. Always good to have closure!!

First off, hopefully you already know this, but I want to make sure you understand that I wasn't chastising or anything with the wording. I was just trying to standardize on the language with what is in the FSM, etc. Sometimes with things (like the altitude corrector), the function is also the name and can help with troubleshooting. So no prejudice intended, and hopefully none taken. So I'm no expert on the BCDD device, but I do know they made several different varieties over the years that all did the same thing (which is limit the intake manifold vacuum at a certain level and not let it decrease below that level). Think of it as your air compressor outlet pressure regulator. Same concept. In 75 (and I think 76) they used that sideways lock screw to hold things in place, and in 77 and 78 they went to an external lock nut instead. Also in 78, they offered two different versions, one for CA and one for everyone else. The CA version was pretty much the same as they had been selling all along, but now starting in 78 the version to NON-CA cars was a simpler version that no longer included the altitude corrector. I guess they decided that other states didn't have enough altitude changes to warrant the compensator anymore. Point is... There might be some small variations between one and another depending on the year of manufacture. If both the units you have use the sideways lock screw, then they are both from 75 or 76 and should probably be pretty much identical inside (including the springs). Can you take a pic with the device disassembled and the parts laid out roughly in the position of assembly?

Matt, The two thin flexible rubber "gaskets" are actually called "diaphragms" and they are a little specialized. You can't just replace them with any simple rubber, so I would treat the originals gingerly. Just wipe as much of the oil off and reuse them. The third gasket (where the BCDD mates to the throttle body) I think is just regular gasket material and can be replaced with a home made version, but the other two cannot. The "adjuster coil" is actually a hermetically sealed air chamber that acts as a little barometer and compensates for changes in atmospheric pressure. Datsun called it the "altitude corrector", and it's meant to account for changes in pressure due to altitude. As for assembly, everything should be put together dry. In fact, there shouldn't be any oil anywhere in the BCDD, but as mentioned earlier, the oil gets there accidently (and probably unavoidably) through the PCV system. I don't think you can prevent all of it. Clean everything off, dry it out, put it back together, and hope for the best?

Wow, those are some fatties!! I've never seen anything like that before! Boiled peanuts are definitely a regional thing. I like them, but you never ever ever never find them this far North. People look at you with a complete blank lack of comprehension or disbelief. Sometimes even disgust. Every time I travel to far enough South, I eat some. My travelling companions think I'm nuts, but that's no surprise. So... A rock solid frozen bag packed in a little box made out of a couple layers of Styrofoam wall insulation would probably survive two days in the mail, wouldn't it? Build a little cubic foot hollow box out of the stuff?

Cool. I think the recommended way to handle it is to set the idle speed where you want it when the engine is warm, and then just deal with nursing the low idle until it warms up. That's how everyone did it in the "Ol' Days". Don't you remember the good old days when you just didn't even try to leave your driveway until you had sat there for a minute feathering the gas pedal to keep it running as it warmed up a little?

Is that a LED shop light you're using in that last pic? That's a really weird effect. Those multiple hard edged shadows give me a headache! Help meeeeee.... Please hellllp meeee!!

Right. All the cars have some way to purposely hold the throttle open a little more when the engine is cold. One way or another. On the 280's it's done with the AAR. On the 240's and 260's it's done with the cam on the choke linkage that pushes the throttle open a little bit. If you don't employ some technique like one of those, your idle will be lower cold than warm.

I would assume that all depends on where you set the temperature. I'm thinking I would want the temperature of the fan switch some token amount higher than the thermostat temp. If the system can operate in equilibrium without the fan running, then you are getting enough heat rejection from just natural airflow through the radiator. But if the temperature of the engine starts to creep above the thermostat temp, then it means the system has gone out of control and you have exceeded the heat dissipating capacity of the radiator (under the current conditions). And at that point, you need more flow. Either more water, or more air. If you are already maxed out on water flow (thermostat is all the way open), then you need more air. Turn the fan on.

Nice work! I've traced most of my carb problems to one of two things... Ham fisted previous owners screwing stuff up or sitting with gas in them for long periods of times. Or both! I've found that surgically clean and actually assembled correctly goes a long way to fixing a lot of issues! Go figure!! About that flat head screw... What are the chances that someone tried to create a ported vacuum source? Is that screw hole close to the butterfly?

Oh, and I forgot something... The oil weight in the carbs should have nothing to do with idle speed. Lean spot in transition off idle? Yes. But steady state idle speed? No.

I'll be the dissenting voice and say that I believe it is completely normal for the idle speed to increase as the engine warms up. All other things held constant, I think this is normal. I have a 280, which is supposed to have a device called the AAR who's purpose is to raise the idle speed when the engine is cold. I took my AAR off, so I have nothing to raise the idle. With no device to purposely bump up my cold idle, when I first start my car it idles a little low at 500. But then over the first ten minutes of warmup, that idle gradually creeps up to 750-800 as the engine becomes warmer and happier. Engines do not want to idle. And cold engines really do not want to idle. Unless I'm missing something here, I don't see the problem! Seems completely normal to me.

That's a great point. And I would go one step further and say that the sensor should probably be on the engine side of the thermostat. You know... the side with all the other sensors on it?

Sorry, but I'm lost. I don't understand all of the distinctions being made with the terms above. Duals Duals all the way to the back Twice pipes Split pipe I understand that all of these options eventually end up with two tips sticking out the rear of the car, but how many mufflers are used, and where does the separation between the two sides start?

I consider that a more philosophically complicated question. You're essentially saying "Everything in the car is new with zero miles on it except the odometer. With that in mind, should the odometer read the old original mileage, or can that be changed to zero miles as well?" I think the answer is "No, it should not be changed and it should reflect original mileage".

Well from what I heard, this guy's cousin is a pack-rat and has a file folder containing years of annual DOT registration records, annual state safety inspection records, and annual insurance documentation that all show the steady progression of mileage from when he purchased the vehicle until now. He can also answer the following questions completely honestly with his head held high: Q - Is that the original odometer in the vehicle? A - Yes. Q - Not considering any potential discrepancies which may have occurred without your knowledge and before your ownership, is the correct mileage of the vehicle what is shown on the odometer? A - Yes. Q - Are you sure? A - Yes. Q - Are you double dog sure? A - Yes. Q - Do you have documentation to prove it? A - Yes. Haha! I think he should be OK. Here's to hoping the DOT has better things to spend tax dollars on than pursuing that dead end!