SteveJ

I see some corrosion, but for a car that's about 50 years old, that's not unusual.

What do you know? Another thing on my roundtuit list that I completely forgot about. Thanks for following up with Shawn.

Thank you, Shawn. @The Unobtainium

That is the EFI fusible link. I can't talk to one side being plastic, though.

I also forgot to mention, you should hear a click when going to ACC.

Be careful. There are two relay coils, and hearing a relay click doesn't mean the contacts are good. Check the fusebox for voltage where I indicated to verify. (Note: Ideally you want to load test as the presence of voltage does not mean the circuit can handle current, but it's good enough to test for voltage right now.)

If you find that the ignition relay has given up the ghost, you can make your own replacement with easily acquired parts or buy them already assembled from ZCarDepot. https://zcardepot.com/products/starter-relay-upgrade-replacement-280z?_pos=1&_sid=fb635bdb1&_ss=r

Many circuits in the fuse box get power from the ignition relay. Are you sure that relay is functional? It is found on the passenger kick panel. Since it is mounted upside down, it is prone to water infiltration/corrosion over the years. The fuses in the red box only have power with the key in ON and ignition relay functioning. The fuses in the green box only have power with the key in ACC or ON and the ignition relay functioning.

I knew I could count on you, @Captain Obvious

I suggest using a marine type maxi fuse holder. Littelfuse makes a line that I believe would work well. It's the Maxi 152 series. https://www.littelfuse.com/products/fuses-overcurrent-protection/fuse-holders-fuse-blocks-accessories/fuse-holders/in-line-fuse-holders/maxi-152 I am thinking it would be a part number like 01520005Z. I attached the cutsheet from Littelfuse. Littelfuse-MAXI-152-Series-Datasheet.pdf To crimp the terminals properly, you would need a set of crimpers like this: https://www.amazon.com/dp/B0978S514B Are you sure the voltage regulator has been jumpered out? You can verify with a multimeter. Measure voltage to ground on the yellow wire at the T plug on the back of the alternator. If it has voltage, the regulator has been jumpered out. However, if that's the case, it would cause problems when the car is running, like human sacrifice, dogs and cats living together... MASS HYSTERIA! Okay, maybe not that bad, but you get the picture.

Is that at a Publix? If not, @Yarb will never see it.

And don't forget that if you need to resort to a 4-speed bell housing, I have one just up the road from you. You just get stuck with the broken transmission attached to that bell housing. 😁

If you disconnect the tach, and the car runs, it would mean that the tach isn't in the ignition circuit. The other things to check are the power and ground to the tach in the dashboard.

That, I can't answer for you.

If fuel is actually leaking from the damper, it's probably toast. Fortunately, there is an aftermarket replacement. https://zcardepot.com/products/fule-injection-pressure-pulse-damper-efi-280z-280zx

If you disconnect the tach, does the car still run?

The first thing I have to ask: Did you verify you have voltage at the horn when you press the horn button? I had to deal with a fair amount of corrosion on the horn tab in my 73. Also, do you hear the horn relay click?

It's less wiring to use an inertia switch. You do have to watch out, though. Many inertia switches like this one: https://www.amazon.com/dp/B08BCJJYGC are wired to Normally Open. You have to move the center wire to the end. That can be tricky if you don't have a good de-pinning tool like this: https://www.vintageconnections.com/products/2-8mm-terminal-extraction-tool?_pos=4&_sid=72ecbc10d&_ss=r (or if you think you'll be doing more wiring, you can get a set: https://www.vintageconnections.com/products/terminal-extraction-tools?_pos=1&_sid=72ecbc10d&_ss=r)

Another, safer thing to do is to get an inertia switch to go with that connector and wire the switch to the circuit and mount it to the firewall. If you get into an accident, the inertia switch will cut power to the pump.

To wire it up, get a 2-pin, non-latching connector. The easiest place to find it is here: https://www.vintageconnections.com/products/6-3mm-connectors?variant=46131501138241 Then you can make a jumper to connect the BW and G wires.

EC section of the FSM. I would not be surprised if your car lacks the parts listed on EC-7. In operation they prevent the butterflies from slamming shut, thus limiting the unburned hydrocarbons in your exhaust. See if easing off the gas instead of just lifting your foot off the throttle reduces/eliminates the backfire.

Bel-metric has 30mm studs. BelMetricM8 Double End Stud Class 10.9 Hardened SteelBel-Metric sells metric hardware & specialty automotive supplies worldwide. Visit our website to purchase metric fasteners, nuts, bolts, screws, washers & more.Also try a Google search on M8x28 exhaust stud.

Yes, there is a separate harness for the fan. Here's the ebay link to that picture: https://www.ebay.com/itm/315467528891?chn=ps&mkevt=1&mkcid=28&google_free_listing_action=view_item Nissan part number: 27155-N3600. https://www.carpartsmanual.com/datsun/Z-1969-1978/electrical/car-heater/from-aug-73

There are plenty of places to get brackets. Here's an example: https://skillard.com/products/datsun-260z-280z-bumper-brackets-for-240z-bumper?srsltid=AfmBOoq0X1jD3JoWPMdb-EqKyvr_f3jRq309BN7hAEXtI-hEXyU80R5Z Just search for 280Z bumper conversion brackets

I see a cable linkage that pulls down at the left arrow. And the front two ITBs are connected to the middle two ITBs at the right arrow. And there is a list of options on their site. I'm not familiar enough with the Jenvey setup to explain the first two options. The third looks like a throttle-by-wire setup. I just found another picture that may show the double linkage.