Zed Head

Not really clear what you're saying here. No offense.

Spiders. Chipmunks. Mice. Carports are outside. Could even be throttle linkage. Make sure all the moving parts move like they should and that there's nothing blocking air flow. Check the inlet tube to the air filter too.

Another random thought. If the AFM is old the carbon trace could be bad. Maybe there's a certain point of the vane travel where the AFM sends bad data to the ECU. One of the AFM tests is to run it through its travel and watch resistance or voltage. Can't remember which. Found it. It's in the FSM but not the Guidebook. Page EF-51 in the 76 FSM. Edit - you can clean the trace with contact cleaner. Hose it down, move the vane. See what happens.

So, you have some inconsistencies, which are probably clues. I just reread your first post and see the vacuum reading. That's a good number. Your fuel pressure at idle should be down around 28 psi. I've had that on mine. Maybe you have a bad fuel pressure gauge. Not uncommon. Maybe you have bad FPR. If it's original the diaphragm can stiffen up. There's a big hose under the intake manifold that connects to the side of the block. That's the main PCV hose. They can fail. Can you describe the problem better. "It accelerates up to about 2500 rpm and then it falls flat on its face. " is not very much. Is it always right at 2500? Does it make any noises when it fails? Does it recover immediately at 2300 RPM? Stuff like that. The suggestion to watch the pressure gauge while driving is a good one. Might be something weird like losing pump power. Under acceleration fuel pressure should stay up around 36 psi.

It's kind of funny how automotive mixes up so many different types of measurement, but you can calculate the fuel pressure drop from the intake manifold vacuum, and vice versa. A good running engine will have about 16 inches of vacuum at idle. Convert that to pressure and it tells you how much drop the FPR should cause. About 7.8 psi. 36.3 - 7.8 = 28.5 psi. 14 inches gives 29.4 psi. 32 psi correlates to 8.8 inches of vacuum. Not very good. Could be valves, timing, vacuum leaks. Just more clues to work with. https://help.summitracing.com/app/answers/detail/a_id/5306/~/engine-vacuum https://www.onlineconversion.com/pressure.htm

I think that when he said 32 he meant with the engine running and intake vacuum applied to the FPR.

The best way to test the EFI system is with a meter at the ECU connector. What the computer sees is what matters. Your fuel pressure readings looked in the ballpark, but 32 at idle suggests low intake vacuum. The EGR system is often the culprit there.

I would set the AFM back to where it was. What happens when you increase RPM under no load? When the engine "falls on its face" what is the tachometer needle doing? The tach needle is a good way to see what's happening with your spark. Does it pop back through the intake and buck when it loses power? If so, try the coolant sensor tweak. It's better than the AFM adjustment. https://www.atlanticz.ca/zclub/techtips/tempsensorpot/index.html

It should do the trick if the internal wall is intact. If it was in my garage I might rig up a hose to that hole and blow on it to see if it's sealed. A simple way to tell, I think.

Big nipple inlet, little nipple outlet.

The cam lobes that have a shiny line all the way around the lobe are probably too tight. The portion of the lobe with no ramp should not be contacting the rocker arm pad. That's where the lash, or gap, is measured. I'd recheck all of the lash measurements. Hard to tell on your spray bar. The head bolts and various crevices are all full of oil. Probably fine once the engine starts. Your cam doesn't look bad considering the time that it's been running. Once they start to grind up the lobes it happens pretty fast. Disconnect the negative terminal of the coil and the sparking will stop. Somebody just posted about blocking an EGR and I remembered the problems people have had with internal EGR leakage. You might check your EGR passages to be sure that they're actually blocked off. Might be one of the mystery sources for some of your problem.

The block-off plate on the top is supposed to seal that passage from the one that connects to the intake manifold passage. But sometimes they rust at the sealing surface or between the exhaust passage and the intake passage. Shine a light down there and see if it's solid. Check the top sealing surface. Or just put a plug in it just in case.

https://spicerparts.com/parts?category=2 Click the "e-catalog" and it takes you to Dana parts. https://www.danaaftermarket.com/search?cat1=LVB&from=20&size=10&y=1976&mk=Nissan&md=280Z&ymmType=AUTO

I haven't fleshed out all of the details but I think that Dana bought Spicer. So, if you can't find Spicer it might be that Dana retired it. You can get Dana parts though. It's weird that Rockauto doesn't carry Precision. For the miles you expect from them, and the agony if they're of poor quality, I'd spend as much as necessary to get the best. https://www.dana.com/newsroom/press-releases/dana-introduces-spicer-dura-tune-center-bearing-for-smoother-quieter-operation-for-downsped-drivetrains/ https://www.rockauto.com/en/catalog/nissan,1976,280z,2.8l+l6,1209226,drivetrain,universal+joint,2392

It looks like you are past Nightmare though, the engine runs and revs and it seems like you're close. Kind of in to recurring weird dream territory, like the one where you're naked at a job you used to have and nobody is really sure why you're there. You guys have that one, right...?

Here's an old thread about wipe patterns. It's an enjoyable topic.

You mentioned overtightened valves. That is very bad. It will make the engine run rough and if you do "burn" a valve, the valve is ruined. You'll have to remove the head again. So, I'd get on that right away. Check your rocker arm wear pads and see if you can tell where the cam lobe is riding. Wipe pattern. Also, I don't know how you chose 18 degrees for initial timing. 10-14 is a common range for people trying to get a little extra power. 7 is the spec., I think. 18 is high.

Not okay. The ports at the throttle body are "ported". They don't get manifold vacuum until the throttle blade is opened. A good one is the tiny one that feeds the AC bottle, or the larger one that goes over to the charcoal canister. Both sitting right on top of the intake manifold and enter it directly. That's where you want to measure. Intake manifold vacuum.

No reserve 71. $16,250 is not enough. https://bringatrailer.com/listing/1971-datsun-240z-128-2/ https://bringatrailer.com/listing/1971-datsun-240z-128/

Edit - forgot that you checked the advance mechanisms already. So this might not be a factor. You should have a steady timing light though. Have you had the breaker plate off of your distributor? Might be that your advance springs are gone. Could also explain the problems with timing if the distributor goes to full advance as soon as it starts turning. Then the weights bounce off of the slot ends with no spring force on them. Here's the guts of a ZX distributor, about the same as a Z's. https://www.atlanticz.ca/zclub/techtips/distributorrebuild/index.html Most of us end up looking at about every single part of our cars before we get them sorted right. They're old and worn.

This is actually a possible sign of the cam lobes slipping off the ends of the lash pads. There's a thread on the site about that. That person got lucky and his cam was not destroyed, but it all started with noisy rocker arms. The fact that yours were looser when you checked again is also not a good sign. People put tens of thousands of miles on their engines with no wear.

Put "wipe pattern" in the search box. Or buy Monroe's book. It's an important part of setting up a cylinder head. You chain is looser also because you had the head shaved. If you have the valve cover off again you might shine a light down at the tensioner to see how far extended it is. They have been known to pop out. https://www.amazon.com/Rebuild-Your-Nissan-Datsun-Engine-ebook/dp/B006VUICHG

You have red plug wires? There's a B and An E molded in to the back of the alternator case. You can see them in your pictures. The thick white with red stripe wire is on the B (Battery) stud. The silver can (condenser/capacitor), and the black wire, are on the E (Earth) stud.

How did the cam lobes look? Did you check the wipe pattern after installing the camshaft? You said "reinstalled and so on", but.... Your cam is in the "after adjustment" spot, which is good. It does indicate that your chain is worn but cam timing should be right.