Zed Head

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.

The engine sounds kind of "cammy". Confirming cam timing and ignition timing will help. I wonder if your damper is bad. Your ignition timing might not be where you think it is. Feels like this has been discussed, not sure. Make sure that you pull the straight side of the timing chain tight when you set the timing mark on the damper to zero. Get that tight then check your cam sprocket notch and groove.

You should describe this in more detail. Surging?

Actually that's the mechanical advance. When you twist it the advance weights move out against the springs.

The hose only gets vacuum when the throttle blade is NOT at idle. When the throttle blade is closed the vacuum is shut off. That's why you don't feel any suction at idle, but get more advance when the hose is connected and you give it gas.

Young people and COVID. Thankfully, I'm not at risk. My young days are past. https://www.yahoo.com/news/son-of-top-biden-science-adviser-suffers-from-long-covid-164911350.html Some people appear to experience long COVID even after only a mild bout with the disease. The phenomenon seems to have lingering effects on cardiovascular health and cognitive function. Patients also say they experience fatigue.

Bad news for Brits. Thankfully, the US is safe. https://www.theguardian.com/society/2021/may/18/any-amount-of-alcohol-consumption-harmful-to-the-brain-finds-study

England is open again! COVID is gone! https://www.gov.uk/guidance/covid-19-coronavirus-restrictions-what-you-can-and-cannot-do Gathering limits have been eased. Outdoor gatherings are limited to 30 people and indoor gatherings are limited to 6 people or 2 households (each household can include a support bubble, if eligible). People can attend indoor and outdoor events, including live performances, sporting events and business events. Attendance at these events is capped according to venue type, and attendees should follow the COVID-secure measures set out by those venues. I'm sure the NFL and MLS will be the same. They obviously don't know about the Skagit Valley choir. Most aren't even trying to fake it with a chin diaper. Plus the India variant. Karma?

You can do a lot with a timing light. Connect the light, start the engine, watch the light move as you turn the distributor. Disconnect the vacuum advance hose and rev the engine. Timing should advance (more degrees on the damper pulley as you watch the light) as RPM increase. Let the engine run at a speed slightly above idle (so that the ported vacuum will be applied) and reconnect the vacuum advance hose. Timing should advance considerably once vacuum is applied and drop when the hose is disconnected. Basically, you'll be leaning over a hot noisy engine watching the timing light flashes on the damper pulley while you work the throttle and disconnect/reconnect hoses. It's fun.

https://www.mcmaster.com/battery-chargers/