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That guy has some cool cars.

From what I've read they figured another, way maybe better design weather stripping? Seems like they quit using them about the same time as the smaller spare tire and larger fuel tank, raised hatched floor and I believe they added anothet hatch lifting strut rod. Not sure on anything but my 9/76 doesn't have them but has the other stuff I mentioned. These cars are like adopted pound dogs. There's a lot of uncertainty that makes them special.

That filter right out of the tank should be full just from gravity. Take the gas cap off and see it gets full. You can also blow air in the filler hole by covering it with your hand and sticking the air wand between fingers. That force a little pressure on the tank.

You have the smaller return like @Captain Obvious shows in that thread up top. It's in the fuel rail, the FPR (fuel pressure reducer) You could take the input side from the fuel tank and put it in a jug, run the electric pump and see what comes, volume wise. Hell suck on it. you'll see the filter fill up before you get a mouth full.

But that would clog the PCV hose. There's 100s sitting in storage buildings that will never see daylight again. I would check ebay or ask members on here.

I wanted to say if you did what that dvd shows, turn the lids upside down, you'll never get close. You can by lids right side up, blow through the fuel in bung and measure until you get it to shut off at .55". You'll need to do it in front of a mirror and it's still hard to do. The new needle valves we get today aren't able to hold the floats weight upside down like the Grose valves did when they made that video. I got halfway up Cheaha mountain when my front bowl ran out of fuel. It was running on 4 5 and 6 cylinders. I rolled it backwards perpendicular to the road and spun it until the bowl filled back up. Had to do that 3 times maybe until I got to the top of the mountain. I was so mad when I got home, finally, I figured out a way to mimic the carb float bowls and got them dead on. I try to get it to run bad but no way. Carbs that are right are unbeatable. This is what I read that gave the right ideas...

Zeddsaver.com https://www.zeddsaver.com/collections/carburator-parts/products/datsun-240z-fuel-pump-spacer

Thats the one with phillips head screws on top. I've been trying to get one for years but I can only get the "ufo" looking stamped metal one. They work good but I'm on #2 since 2015. Both from o'Reilly. They were the only local pickup I could get. I beleive your's is a GBM maybe? It's the closest to OE I've seen. Ztherapy.com will have the spacer you need.

I couldn't get the .55 dvd on SUs to work. I took the lids off and put them on top of a small glass with the floats moving up and down free. They are finally doing as when new. Here's the measurement I used from @zKars if I remember right.

Okay. Option #1.

You can buy gasket paper at chain stores but a good idea I learned from here, cut one out of a cereal box.

If it was slower I'd say stripped groves in the plastic cog but being twice as fast??? I'd pull the connection/housing out of the transmission and give it a good look over. That's the easiest thing to do that I can think of. Good luck. And it's the same height as the fill hole so you won't loose any fluid. Jack it up from the passenger's side to move the oil over even further away.

Make sure your dipstick numbers on the side start with an E. That's the right length for the 240. The 280s start with P. Oh yeah! Back to the first. Do you add oil before an oil change? If so it's using oil. Fill it to the FULL line at the top and check it every week to see if it's lower.

Normally efi isn't effected by the gas pedal on start up. Not like a carbed car. I wonder if the light gas peddle push is enough to open the throttle blade allowing more air? I'm not sure how ported/nonported effects starting. If it had already been talked about you need to try opening the big phillips head bypass screw in front of the dashpot. It's got a spring around the threads.

Your numbers look good to me. My first thinking was lower spark on #4 but you've tried everything. The valve clearance on #4 may be off. The locking nut could be bad or something with that one valve? When I fouled plugs all the time on a 2 stroke dirt bike I had we put an anti foulet on and helped from what I remember. It was a 1982 motorcycle too. I do not know if they work on cars? Maybe someone else can explain? https://www.google.com/search?q=spark+anti+fouler&source=lmns&bih=449&biw=320&client=ms-android-americamovil-us-revc&hl=en&sa=X&ved=2ahUKEwiu6ZqO08X9AhUFrIkEHfBMAxcQ0pQJKAB6BAgAEAU

I've seen some nice collections displayed as art in guy's offices. My dad had 1957 until he lost his man cave when my lovely mother divorced him. He was so angry he left them up on the walls and the property was sold. I was too young to realize the value. He used them to repair rotted gutters and holes that squirrels had eaten into the sofits. I've been late to the hobby but never throw mine away and buy them at yard sales and trade day events. One day after I'm gone hopefully the new owner will keep it going. I've picked the nephew I want to get my stuff but it'll be a problem. He has 3 other brothers.

I watched the video and it shows the screwdriver trick to hold the carbs open. That'd be the easiest thing to do.

Big help setting valve lash by yourself while they're hot. Hell when they're cold too! I do mine cold now after a few years break-in.

You might find one in these threads? A lot of people remove them and just feather down the throttle by downshifts I suppose. Mine works but I do think I've read they are adjustable. Seems like you run them out further on the threaded section to get up against the plate. It's been a long time for me but good luck, I think you can figure it out. Here's the threads with the info... https://www.google.com/search?q=280z+throttle+dashpot+replacement+classiczcars.com&client=ms-android-americamovil-us-revc&sxsrf=AJOqlzVsrRCCr4v1H1MhXQyklV95Oob-xQ%3A1677801463922&ei=9zcBZIbvN-W8ptQP4bqU8AM&oq=280z+throttle+dashpot+replacement+classiczcars.com&gs_lcp=ChNtb2JpbGUtZ3dzLXdpei1zZXJwEAMyBQgAEKIEMgUIABCiBDIFCAAQogQyBQgAEKIEMgUIABCiBDoICAAQogQQsAM6BwgjELACECc6BQghEKsCSgQIQRgBUMMQWIhzYIeCAWgAcAB4AIAB9gKIAZcUkgEHMC45LjMuMZgBAKABAcgBBcABAQ&sclient=mobile-gws-wiz-serp

I found front and rear plates from 1960 something in the ceiling of my garage. The guy that built the place was a mechanic and must have thrown them up there. Anyhow I traded both of those to Dr Tag in Georgia for a pristine 1972 Alabama tag, did the initial pay and paperwork now it's like a vintage tag. I won't have to pay another nickel for that car's plate nor the '77 280 which has the standard vintage plate as well as my '87 hardbody pickup. They get me for way more but I jab them when I can.

That's the throttle dashpot. It keeps the throttle from slamming down to nothing after letting off. https://www.classiczcars.com/forums/topic/39698-throttle-return-dashpot/

@wal280z THE guru for that type stuff. I have his upgrade "from the junkyard" Cadillac parts. How are you Wayne? Long time no speakie. Hope you are good.

$104,000 with 5 days to go. $150,000 is my guess. You want to play?

You might figure a way to put a screwdriver through the loop on the throttle rod and keep it pried open?