Captain Obvious

Well then you haven't read many of the things I've written. Haha!! But regardless, it's clearly an experience issue here, not ability. You'll get it!

So getting back to the current issue at hand of your idle being high even with the adjustment screws being turned out. Another remnant of your previous carb conversion is the (vertical) linkage rod that pushes down on the center linkage... That rod used on your original carbs (flat tops) is a different length than the rod used for your current round tops. Most people replace the original longer rod with a correct one, but it appears your previous owner modified your original rod instead. Looks like he cut one end off and attached a loop to the top. "Welded" maybe? In any event, it looks like from your pics that linkage is already slack, but just to make double dog sure, you can pop the plastic ball off the linkage at the bottom. That way the center linkage between the two carbs should be guaranteed to be loose and not affected by anything else upstream. Here's a pic of what I'm talking about. You should be able to use a small screwdriver and pop the plastic female end off the male ball:

Haha!! Figured. And don't feel bad about it. If you have no experience with those carbs, you just have no experience. So when they said to loosen the linkage, the intent is "Make sure the center linkage is not actuating either of the two carburetors." Or put another way... "Loosen enough of the adjustment screws to make sure the linkage is not holding the carbs open at all." So in the interest of teaching a man to fish... You currently have four adjustment screws. The original system had five, but because of the previous carb conversion that has been done to the car, one of the five original screws does not exist. The one that does not exist is the "fast idle adjustment". Don't get worked up about the non-existence of that screw as I believe it's only function is to temporarily raise the engine speed while checking carb sync. Once the carbs are synced, you loosen that screw and it does nothing until the next time you want to check the carbs. So the remaining four screws... Two of them are idle speed screws (labeled in my pic above) and then there are two others. The front screw of the two others - You will notice that since you removed that front top spring, the piece that screw is attached to is loose and flopping around. The reason for that is it originally connected to a piece of emissions equipment which has been removed from your car. That screw and the floppy linkage piece it threads into now do nothing. Forget about it. Many people zip-tie it to the linkage between the two carbs so it isn't in there flopping around, but even that is not necessary. Then the fourth screw - The one on the center linkage towards the rear. That one is your sync screw. AFTER you get your car idling properly, you use that screw (in conjunction with the fast idle screw) to make sure the carbs are sycned ABOVE idle. KInda hard to describe some of this with just words. If you want, I can add some more annotations to the pics.

Yeah, I was thinking the same thing about the butterflys. It sounds like the front one is not closing all the way. Either due to an external linkage issue, or from an internal issue like the butterfly plate has been removed in the past and was not reinstalled correctly. And I'm not sure what manual you were looking at that recommended loosening those nuts, but it's neither necessary, nor a good idea. I'd get those tightened back up. I'm thinking it was a misinterpretation of the procedures. Something like maybe you read "loosen the linkage between the two carbs" and your interpretation was to loosen those two nuts? If that's the case, then it was a misinterpretation, and you should get those tightened back up.

I've measured a couple stock heads and the original thickness of the N47, the P79, and the P90 is 108 mm (4.255 in). You can use that 108 mm measurement to determine if a head has been cut previously or not. I haven't measured the other head varieties (because I haven't had them pass through my hands) so I cannot tell you what the others are, but I suspect they are all the same.

Yeah, I figured. It really sounds like a cool mod, and it's been forefront in my mind recently because my clutch is squeaky and notchy. I'm sure I need some attention down there.

I assume if you had pics, you would have posted them?

And just to make sure we are talking about the same screws... You're saying you loosened up four screws (including these) a whole bunch and you are still idling at 2500 RPM?

So here is an annotated pic showing what I was talking about above:

Yup. That's the one I referred to as "the top front return spring kludge". Much easier to identify with your annotation on the pic. The way is right now, it's pulling the throttle ON instead of pulling the linkage off.

Seeing your pics, you do have the return springs on the carbs (hooked down onto the heat shield), so that's not it. With a quick look, I do see some stuff that isn't right though... The long hex nuts that go onto the throttle butterfly shafts are loose on both ends. The front carb is loose, and the rear carb is missing a lock washer and is loose. The top front return spring kludge appears to be pulling the center linkage in the direction that would increase throttle position. If it's fighting against the front carb return spring (and winning), that will open the front carb and raise the RPM. You shouldn't need that spring at all. I'm not currently in position to annotate pics, but I'll do that later tonight unless someone beats me to it. And also... Yes, the cables are choke cables. When pulled, they also open the butterflies some to raise the idle speed. So it would be important to make sure the cables aren't pulling the carbs open.

I have vague recollection of someone here putting roller bearings into their clutch pedal to get an uber smooth fantastic pedal feel. I'm gonna test old brain cells and tag @zKars maybe, for possible pics of this process?

Your plug installer tool looks great. I spent way too much time making a custom application device on the lathe. Don't have any pics, but when I use it, I'll snap some. Yours not only took a lot less time to create than mine, but it's also adjustable which mine is not. Forest through the trees....

Haha! If you're anywhere near as good at counting the beans as you are with the stuff that you really enjoy, then you must be pretty good at all of it! I'm not worried at about you opening up your first motor.

I'm pretty sure you know this already. but one might point out that the order of operations here is extremely important.

I thought you were a numbers guy, not an electronics guy? I mean... Seeing your place, one would never guess that, but isn't that the case?

I think that knowing what you're up against (literally) and hearing about the experiences of people who have drilled beyond the plug should be enough. Just be careful and pay special attention to when the drill bit breaks through the other side of the freeze plug. It's only a sixteenth of an inch thick.

Your original carbs (the flat tops) had integral return springs built into them. Your current round tops do not have integral springs and require two external return springs (one for each carb) instead In normal round top configuration, those return springs attach to the heat shield mounted below the carbs. Do you have those return springs installed? Can you take some decent resolution pics of your carbs and post them here for scrutiny?

Be very careful dealing with the front plug. There isn't a lot of room behind it and if you just try to pivot it in place by knocking one side inward, there isn't enough room to swing it to get a grip on the other edge. So because of that, sometimes people drill through them and run a screw or something into them to act as a handle. HOWEVER. More than one person I know has damaged blocks by drilling through that front freeze plug and kept on drilling into the cylinder jacket. Don't do that. I drilled through mine (very carefully paying special attention to when the drill broke through the other side) and then I tapped the hole and used a piece of threaded rod attached to my slide hammer to yank the plug out. Looked like this:

Yeah, I don't know what the deal is, but the number and size of the core plugs is an issue. Seems like it would be simple, but it's not. Everyone seems to think that the same kit fits everything from 70 through 83, and that's just not the case. Here are my (relatively disorganized) notes about freeze plugs: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ On the F54 block, there is a total of eleven freeze plugs required. I had originally said there were ten, but I edited this post to show there are actually eleven. Nine 35 mm One 40 mm One 50 mm I bought (a quantity of two) Sealed Power part number 381-8027 sets off rockauto Jan 2020 Each set contains 1-50mm, 1-40mm & 7-35mm plugs. I had to buy two sets to get enough of the 35mm plugs. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ However, I just took a look and that Sealed Power number is no longer available on RA, and actually has a very low web presence anywhere. I still have the bag un-opened somewhere and I will snap a pic or two when I find it, but it could take a while. It would probably just be easier (albiet more expensive) to buy more than one kit of the other more easily available kits. You'll have plugs left over from buying multiple kits, but at least you'll have plugs.

I took a quick look at the parts diagrams and couldn't find the vacuum water cokk, so I can't help you with the part numbers. I can answer the other question, however... The water cokk is controlled by the temp slider lever allowing you to control the temperature of the heater core. The vacuum cokk shuts off ALL water to the core whenever the system is switched to A/C (regardless of the temp slider position). Once in this mode, the slider no longer controls the water through the core (since it's off completely), but instead controls the temperature switch that cycles the A/C compressor. So, yes... They are in series, but they serve two related, but different functions.

And about cleaning out the block... I made a little adapter for a garden hose that shoots a thin jet and I was able to stick it deep into the core plug holes and flush junk out. Kinda like a power washer, but just at tap pressure. Looks like this:

That's the same head gasket I went with as well (11044-P9600). I like the round holes for the head bolts better than the large oversize rectangular cut-outs of the previous version. My thinking is that I want as much meat as possible between the water jacket holes and anything else. I got mine from Riley at Lynchburg. Out of curiosity, where did you find yours?

Here are my (relatively disorganized) notes about head gasket stuff. Maybe something in here will be of help. L24 and L26 versions - item #19: (From the old fische) 11044-E3100 11044-E3101 11044-E3102 11044-P3000 11044-P3001 11044-P3000 11044-P3002 11044-P3004 This number supersedes all the previous numbers and is the final listing for the early motors and its used on 240 and 260 L28 versions - Item #18: (from the old fische) 11044-N4200 - No superseding listed on the fische, but was superseded after the fische After the fische from other on-line sources 11044-N4200 - 09/1974 - 04/1980 11044-N4210 - 05/1980 - 02/1981 11044-N4220 - 05/1980 - 02/1981 (don't know why there are two of them. Might be turbo) 11044-N4221 - Supersedes all of the above 11044-P7900 - 80-82 GL (turbo) 11044-P7910 - 81-83 GL and non GL (everything 83 turbo and non turbo) 11044-P7911 - Supersedes all of the above Looks like the 11044-P7911 may have been superseded by 11044-P9600 Part Number 11044-P9600 Replaces NLA 11044-P7900 11044-P7910 11044-P7911 The original gasket was made by NRZ Nippon Reinz This gasket is made by Stone It is for the original head/block combo on 1981 1982 and 1983 S130 280ZX" And if you look closely at the gasket, you can see both the Stone logo and the Nissan logo victor reinz 61-52115-00 (they list the same gasket for Z and ZX)

I dug into this a little bit deciding which gasket to use on my F54 and I decided on OEM. My thoughts on the cooling holes is that Datsun was (forever) chasing unevenness in the cooling between cylinders and kept changing the size and existence of the coolant holes accordingly. And by the time they got the to end of the series (which is what you have), they had completely blocked off some of the holes towards the front of the motor with the intent of forcing more water to flow to the back of the head before heading to the radiator.