Zed Head

On a 76 one would be right around where the yellow is in this picture. An eyelet with a screw through it in to the manifold. The ground wire is in that harness. The alternator also has a dedicated ground wire which would provide grounding through the mounting points, assuming contact was good. Poking around with a meter is always fun. You could also use a headlight bulb to pull some amps through the various ground points. I like to measure things, but, adding a redundant ground wire won't hurt anything if you don't trust your measurements. https://bringatrailer.com/listing/1975-datsun-280z-45/

The premise of the added relay is that something along the circuit to the starter solenoid is drawing so much current that the solenoid can not actuate. The circuit has worked just fine for over 40 years I'd guess. Something is different now. The wire from R to the resistor bypass circuit is one of the few things that are different during Start. You have a "key at Start" voltage drop problem. Just offering troubleshooting paths to check. As far as the relays, there are several pictures on the forum of burnt/baked relay coils. They just get old and crusty with time. It's not just water that causes problems. How has it been starting with the new relay?

I missed the part above where you already installed the relay. Oh well. Don't overlook this part. Part of it is only in use at Start.

Nissan put the relays in an area that gets wet if the windshield leaks. If a crusty relay is causing excessive draw the extra bypass relay might just be delaying another problem. They're by the fusebox. I don't know if Nissan just changed names or did not use a standard ignition relay in 75. The 1976 BE chapter shows one but 75 only shows an "igntion interlock relay". They don't include the small diagrams either. 75 is one of the difficult years. This is the 75 drawing - Here's 1976, where that rusty picture above came from -

Interesting that the draw doesn't affect the other gauges, not even a needle quiver. But it does affect that horn noise. Not sure what that is, is it the seat belt warning buzzer? I have to say/ask - why are you avoiding the simple electrical diagnostic methods? It's a very straightforward circuit with a simple solenoid at the end. Low current, one device. I almost replaced my starter when the spade terminal lost contact but then I pulled back to the basics and figured out what was happening. Another time on a Ford Escort I was halfway down to the starter when my brain told me the symptoms didn't match what I was trying to fix. It was a loose battery terminal (not your case since the gauges don't move). Notice on the wiring diagram that there are other functions on that Starter wire from the ignition switch. You could check those to find the break.

Who is "they"? "Any or all of the above" doesn't mean anything. Z Car Source is out of business, AZC is more of a performance shop. Did you get them from eBay? Maybe you just bought from the wrong place.

The Tokicos are gas shocks also. There are some threads out there about how the gas shocks raise the car about 1/4 to 1/2". It's not much. You can push the shaft in easily by hand, it's probably only 20-30 lbs of lift. Who is Arizona Z Source? Did you mean Z Car Source? Or Arizona Z Car? http://www.zcarsource.com/ https://www.arizonazcar.com/

I downloaded the wiring diagram years ago from 240260280's AtlanticZ site years ago. It is excellent although it is black and white. http://www.atlanticz.ca/zclub/techtips/electrical.htm

1975 seems to have some odd features. The wiring diagram shows an "interlock relay". It exists for both Manual and Automatic. Could be involved. I don't know how it works. I think that the 74 260Z's have it also. @SteveJ knows about, I think. The diagram might be a holdover from the 260Z though. The 75 FSM does not describe it. Either way, your problem seems tailor-made for a test light with alligator clips. Set it up on the circuit you're testing, turn the key, see what the light does. If you get the click but the light stays on the problem is downstream. If the light goes off it's upstream.

I've seen that before too. Contact cleaner or Deoxit ia also a good idea. Deoxit is good everywhere actually. https://caig.com/deoxit-d-series/

If the aluminum stakes/posts/pins are still in the port I'd bet somebody might scavenge an old head for a liner and send it to you. Maybe. Post a picture of the linerless port anyway, if you can, just for viewing. Maybe it's fixable.

30 psi is about right if you have the vacuum reference hose connected to the FPR. The simple pump test is to remove the FPR vacuum hose, or disconnect the starter solenoid and turn the key to Start.

Two similar problems that I've had are a loose pin in the ignition switch, unlikely in your case, and a loose ignition wire connection at the starter solenoid. The last one seems possible. In my case the spade connection seemed to fit well but was not making good electrical contact. I crimped it tighter with a pair of pliers and it was fixed. You could rig up a wire under the hood to supply power directly to the starter solenoid terminal (or use a wrench or screwdriver to jump from the hot cable) and try it when the problem happens. Split the problem in to two parts at the solenoid terminal.

If you break it down to the basics, the whole point of the liners is to stay red hot and complete combustion of anything uncombusted. So, the action in the cylinder should not change. The 280Z exhaust manifolds are pretty primitive if you take a close look. The exhaust ports basically dump in to a large common chamber. I can't see a reason that the missing liner would do anything more than make one cylinder dirtier, emissions-wise, than the others. The EFI and emissions systems were advanced for the time, but pretty primitive today. This is just me, but I'd run it. Probably won't see a measurable difference. The converter will absorb one cylinder out of six's extra hydrocarbons. That's probably the only real effect. 75 and 76 didn't even have liners.

It probably won't have a huge effect. My impression has been that it just isn't worth doing. What kind of testing does "Canyon Country" do? Most only do idle and 2500 RPM. Does the car have a catalytic converter? What does "pretty much fell out" mean? Do you still have it? You could probably reattach it, depending on the failure mode.

I don't think that material quality was very good back in the early days of fuel injection. Nissan had a team of engineers create a document about changing hoses. So, it was probably a thing back then. Much of the garage lore we all picked up is only based on who the oldest, most authoritative guy (best BS'er) in the garage was at the time. Just saying...I learned a lot of not-quite-right stuff.

Other people have noted the lack of instructions with the ZCD parts. He has a good thing going so doesn't seem to care about the small details. If you get the rear brake kit pay close attention to the parking brake cable. Apparently the cables supplied and/or the fittings are of poor quality. Can't remember if the ends pull off or some other part fails but people have had to fix the problems themselves. @Derek has posted about it on Hybridz.org. Good luck with your mother's health issues. Those are tough. It's good to have something to keep you occupied in the free time. The car looks good, I don't think I've seen a full view of it before. I found Derek's post on Hybridz.org. It's the bracket for the sheath. https://forums.hybridz.org/topic/131360-rear-disk-kit-from-zcar-depot-anyone-using-it/#comment-1219981

The AltlanticZ site says 188. I'm pretty sure that the Bosch 028-150-105 (up to -114 or -115) injectors are what the Nissan injectors are copied from (the hose lengths are different in the Bosch series) and there are numerous sites that say 190. Here are a few links. They seem to all be at 3 bar, even though the early Bosch injectors used 2.5. Apparently many of them just copied from other sources. Except the last link which clearly says they did the measurements. They got 222 for the Bosch analog. http://www.atlanticz.ca/zclub/techtips/injectors/index.html http://users.erols.com/srweiss/tableifc.htm https://www.motormanfuelinjection.com/Fuel_Injector_Flow_Rates.html https://injector-rehab.com/knowledge-base/flow-rates/

Yes, a car battery. Along with a pump. Probably took 1 - 2 minutes minutes to fill the bottles halfway. 190 c / minute is the rate I think, or close to it. 1/2 bottle is about 240 cc. It seemed like a long time with all of that vaporized fuel shooting in to the bottle. My first try was in an open garage but then I moved it outside in to the open air. Actually, to be honest, it still feels risky, especially when I tested some injectors with better vaporization than the Nissan injectors, which shoot a straight stream. Be careful, of course. Stay upwind. Use long control wires and a spark free switch.

Have you thought about the internals? I built a flow tester (wide open only) using a spare fuel rail and wiring the injectors in series. Fill a bunch of Coke bottles and compare. I've posted pictures occasionally. The ones that came on my car were way unbalanced but I think that they were aftermarket. I don't know what you're working on but the thought came to mind. Since they're out.

Came across some spray bars being used in a pretty sophisticated modern engine. About 5:05 in the video if it doesn't load right.

Kind of looks like a protective cover could be there. There's a pseudo-hook above.

Was it obvious by eye? The color? Never really looked at mine when I had the head off.

My 76 injector hoses looked okay on the outside but they were split on the inner diameter. You could see the tiny radial cracks. I smelled fuel one morning, pulled over, left the engine running and opened the hood and a tiny stream of raw gasoline shot past my head on to the sidewalk. I had a screwdriver in the car and cranked the clamps down to get home. The engine was cold, otherwise my car probably would have ended up like one of those in AV8's recent thread. On replacing the hoses - I used Gates EFI hose, and gasoline to press the pieces on, but I found that the material "creeps" over time on the clamped end. I had to retighten the hose clamps at the rail a couple of times. If I was doing it again I'd try to find some constant tension clamps.

Found it. It's on page 8.