SteveJ

So I went out to check to see where exactly is the best point to measure ground on the steering column. For the body ground, you can use one of the screws on the scuff plates in the doorframe. Here is where you should put the other probe on the steering column. Be sure not to touch the top ring. As for the body ground, it's in the engine bay. Unless you have a host of electrical issues with your car, the ground should be there. I put a battery cable from the negative to the grounding point because I could. It's on the passenger side of the engine bay, not too far from the firewall.

Do you have any idea if there is a part number difference or just a vendor variance?

You're just not thinking about electricity the right way. Another term for voltage is potential because you have potential (aka stored) energy when the circuit is not complete. Here's a rough diagram of the horn circuit. You go from the positive of the battery to a fuse (not shown) to the horn relay. The electromagnet in the relay is just a long piece of wire. When the horn button is not depressed, there is no current flowing through the wire of the engine harness, dash harness, or horn relay. So you have a long piece of wire attached to the positive terminal of the battery, and that wire includes the red/black wire going to the horn button. Measuring along any point of that wire is the same as measuring at the positive terminal of the battery. The red/black is not at the ground state. When you press the horn button, you are putting the red/black wire at the ground state with the negative terminal of the battery. Now you have current flowing. The electromagnet in the relay is pulling in the contact to sound the horns. You're going from potential energy to kinetic energy because work is being done. So until you press the horn button, you should see voltage to ground at the green/red wire.

As long as you don't have a dash cover glued onto the dash, it's not too bad to remove the tach. It just involves contorting yourself some. I'm in my mid-50s, and I can do it. Heck, I did it a couple of weeks ago when I was helping with diagnosing a car. There are two wing nuts holding the tach in the dash. You pretty much have to lie down in the footwell and reach up to find them . Once you pull the tachometer out enough, you can disconnect it from the wiring harness.

Well, everything I could find about the tach pegging says that the tach is going bad, and for many it's the failure of a discrete component, like in the thread I linked. The good news is this can be repaired. This place fixed a friend's 260Z tachometer about 11 years ago. https://www.bobsspeedometer.com/

Color me unsurprised. There's only so long a manufacturer can support obsolete technology. I'm usually surprised when something is still available.

Hey, @zKars, I think this means you have to show up for ZCON since your car was featured.

I skimmed through the EF section of the 74 FSM. I didn't see that valve, and it wasn't described in the theory of operation on EF-32. I would not be surprised if the vent line from the tank was capped at the tank or in the engine bay.

You're right. I was speculating based upon the dropped performance. That's why I revised to dying tachometer once the voltage readings came back.

He has a 74. I never noticed pressure building up in the tank in my car. Of course, the carbon canister was not hooked up properly.

@Dave WM I think you'll find the part numbers on this page: http://www.carpartsmanual.com/datsun/Z-1969-1978/electrical/electric-unit-switch/section-2/280z. Start around part 31 and work your way down. Be sure to go to figure 2 of 2.

An easy way to see if the steering column is grounded is to check continuity from the shaft where the steering wheel mounts and the body.

The voltages look right for a 240Z with points. Unfortunately that probably means your tachometer has seen better days. Read through this thread: Of course, @beermanpete figured out how to fix his. I wish he was still hanging out around here. As for the fog lights, that green/white wire should only be used to control a relay along with a switch. Don't use that wire to power the fog lights. You would need a fused 12 volt source connected to the 87 terminal of a relay with the 30 terminal connected to the red wire. (Relay explanation here: https://fiddlingwithzcars.wordpress.com/2012/12/22/relays-simplified/)

With the 280ZX distributor you will need to buy a 280ZX distributor mount and timing plate. The mounting bracket seems to be pretty rare now, and it's not like they were very common 10 years ago when I last bought one. (People seem to forget about this when they suggest a 240Z owner swap to the ZX distributor.) A 240Z distributor with Pertronix would probably fit in the mount you have now. By the way, Summit Racing seems to have the best price around on the Pertronix right now. I'm not sure what part number Pertronix you would need for the Mallory. Looking at their website, this was the only 6 cylinder, 12VDC, negative ground Mallory kit I saw: https://pertronixbrands.com/products/pertronix-ml-161-ignitor-mallory-6-cyl

First, locate the vent line in the engine bay. Blow some compressed air at low to moderate pressure into the vent line, and have someone listen at the filler with the gas cap off. The helper should hear the woosh. That also allows you to figure out if someone capped off the vent line in the engine bay. For the plastic panels, you might try some thin weather stripping. https://www.amazon.com/Weather-Stripping-Sliding-Speaker-Dashboard/dp/B07WGTHBSC Or maybe something like this: https://www.amazon.com/Weather-Stripping-Silicone-Window-Transparent/dp/B07XRVBSYC Here's some 1 mm weather strip. https://www.amazon.com/uxcell-Thick-Adhesive-Weather-Strip/dp/B07L6JKJD1

As for the bolts to adjust the timing, there's a picture for that. Here's a YouTube video where a guy made a transitorized ignition with a Hall effect sensor. You'll see early in the video that he passes a magnet over the sensor, and you can see the spark. For the Pertronix ignitor, there are magnets in the ring that goes below the rotor. The ring should be indexed to fit over the shaft only one way. Otherwise, I don't have much to add to what @Zed Head said.

Do you have any loose wires around the steering column? (Aside from the key in switch buzzer)

There should be two adjustment bolts for timing. Have you played with both? If you have, and what you said still holds, here is what you might do. Put the mark at the first line past the middle and toward retard. Consider that your static zero position. Adjust the oil pump from there to put the rotor at TDC. Still this picture bugs me. In the 240Z, I could swear that when the engine is at TDC, the rotor is near the Hall effect sensor, not 180 out like in your photo. I tried searching for images with your setup, and I tried searching Hybridz. I didn't find anything useful. Maybe someone smarter than I am can chime in about this.

No, it is not normal to build pressure. Make sure the vent line is not capped off.

Edit: Some things got clipped out by mistake. Does your distributor still have a plate like this? Here is how it's lined up on my 240Z. Here's what you might try to do before messing with the oil pump. (Minimize jumping around.) Set the car at TDC. Align the mark on the plate to the middle. Verify that the rotor is at the #1 plug. From there, you'll have a better idea of what way you need to go.

Did you retard timing any after moving the wires? It might be too far advanced.

After looking through the wiring diagram, I am guessing that mystery connector is for the fog light switch. You don't have a fog light switch? Well, North American cars are wired for fog lights, but Nissan didn't install a switch. It's just as well. The engineers designed the circuit to come off the parking lights. That circuit already has enough issues without fog lights added.

Please make it easier to provide help. Wiring diagrams differ by year. As for why your tachometer is maxing out, I would posit that there is too much current flowing through the tachometer. Maybe the black/white wire from the tachometer has been damaged and is slightly coming into contact with ground. This would cause more current to flow through the tachometer sensor and less to flow out to the ignition. Maybe you reconnected something wrong, though there shouldn't be anything connected to the black/white wire between the tachometer and coil positive. While the car is running, measure voltage to ground at the green/white wire on the ballast resistor and also measure voltage to ground at the black/white wire on coil positive. There shouldn't be much of a difference in voltage between the two. Also measure voltage to ground at the black/white wire on the ballast resistor. Pleas report your results. After you supply the year of your car, I can look at the FSM to try to identify the mystery connector. I'm guessing you have a 72 since I see short pigtails on the fusebox, but I don't see sockets for bulbs around where the control panel goes.

Tools from this kit may be more gentle on the paint than an old screwdriver. https://www.harborfreight.com/automotive/auto-body-trim/trim-and-molding-tool-set-5-pc-64126.html

Most people use 20wt oil in the dampers. You should check the level in the carburetors. Also, with the car off, try to lift the piston in each carburetor. The damper oil should provide a fair amount of resistance. I'm wondering if the front carb has enough resistance. Ace Hardware stores typically carry 3-in-One brand oil that is 20wt.