DadAndLadZ

I took the spring off, and I do know that the throttle linkage between the two carbs is loose; I loosened both those as directed by all the various manuals when I started trying to individually balance the carbs. It's not cross threaded, it's just screwed all the way out. I will put a lockwasher on the back one, thanks for pointing that out. And more pertinently, yes, I can back the idle speed screw on the front carb all the way out and it's still roaring. It is like 100+ degrees every day here at present, and I also hurt my arm/shoulder over the weekend, so it's likely to be at least a few more days before we get back out to the garage, but I will holler back here as soon as we do. THANKS so much for all the help so far.

Well, I bet that makes a difference! Will investigate, thanks!

Yeah, I don't like having the fuel line right over the top either, will def move it before this is all said and done. I unhooked the choke cables on both sides and removed the spring recommended by @w3wilkes. Here I have both throttle screws backed all the way out and it's idling...check out the difference between front and back when I choke them individually...

There are.

Here is a short video showing the fuel lines:

Oh man, I will check out everything you said. I would be very grateful for you to annotate. Here are some pics of the supply side, as requested above, too.

Interesting, thank you!

The single fuel line goes over the valve cover to a fuel filter, which is connected to the hard line from the electric fuel pump mounted near the gas tank. It pumps pretty good but I have not actually checked the fuel pressure and not sure how to do that? I can tell there is a bunch of stuff missing and/or Frankensteined here but being unfamiliar with these cars in the past, it's hard for me to get my head around what I'm looking at. I really appreciate the insight from you folks... I tried another Z Car forum first and it's like crickets over there! Will probably be the weekend before I can get back out in the garage and fiddle around with it some more, but will def keep this thread going as I move forward. THANKS most sincerely for all the input.

Those are the choke cables, no? I have had the throttle linkage disconnected entirely while doing all this but have not messed with the cables indicated here. Still it makes sense to disconnect them in case it's a choke problem hassling me here, thanks for pointing it out. Also that fat hose goes from the valve cover to the back of the air cleaner assembly, as do the two smaller hoses coming from the carbs. The big one I assume is some sort of PCV thing? Here I reassembled the air cleaner backplate hose layout for a visual aid.

OK, thanks for your patience, folks...here are finally some pix of my carbs as they are. If you see any red flags, I'd sure like to hear from you. Short story for reference: Car runs. Front carb sucks vastly more air into the horn than back carb, even when throttle screw backed completely out and all linkages disconnected. Idles at 1500-2500 rpm at present, with all screws backed completely off the stops so carbs should be at maximum closed position. Cannot even remotely try to balance.

Hey, thanks! Will check this out for sure. We have been busy over the long weekend, one of the great-grandparents passed away right on the Fourth of July and it's been family stuff the whole time. Will get back out to the garage to tinker some more soon — but with tomorrow's forecast calling for 104º F, it might be another couple days or so.

Hey, thanks! I have a Haynes manual and a Z-Car restoration manual too but this thing has been Frankensteined and none of the pictures match what I have on the actual car! lol

Thanks! The older of the two wants to be an engineer, and he wasn't around all the years I was driving old cars, so this is for his benefit!

I will take some close-up pix tomorrow and post here. Thanks, stay tuned!

Yeah it's not there at all on mine!

DUDE THANK YOU... I keep looking at reference material and manuals and they all mention a "high idle test screw" at this position but there ain't any. All the other diagrams show some sort of vacuum biz, too, but on our engine there looks like a blockoff plate there? I am just completely unfamiliar with this engine setup before this so I don't have anything objective to compare it to. This is VERY HELPFUL!

Thanks for your input, this is all very helpful!

Hey, thank you! Will def check that out. And I appreciate everybody here being kind... I have done lots of shade-tree mechanic work over the years but this twin-SU setup is new to me, and I feel like a bozo. lol

Damn, dude, this is exactly the quality answer I was hoping to hear. It will be a day or two before we get back to working on the car but I will 100% check this out. THANK YOU. Even if that's not the problem, it confirms my suspicion that these are in fact the earlier style carbs I have seen described in numerous other posts on the subject and helps me figure out which set of references I should be looking at. I think I actually have disconnected that throttle linkage while it was running, to no avail, but it's all a blur to me now and will def give it a shot next time I'm out there. Much obliged!

Hi, all: This is my first post here. I am a middle-aged dad of two young sons and have owned 40-50 vintage cars, trucks and motorcycles over the years. I mostly taught myself to wrench on them, so while I am generally competent, I have some glaring gaps in my mechanical knowledge. Recently the boys and I picked up a '74 260Z 2+2 that had not been started in seven years, and we have been working on getting it back on the road. The car starts right up and runs but the twin SU carb setup was running lean and way out of balance. I took the tops off, did some cleaning, replenished the oil in the dashpots, reset the mixture jets to the two-turns-below-deck baseline prescribed in the procedures I have been reading and put the Uni-Syn on there — only to find that the front carb is sucking in TONS more air than the back one, and the engine idles at like 2500 rpm with the choke disengaged. Fiddling with the throttle screws, I discovered that even with them backed out all the way, there is no way to drop the engine speed. Can anyone tell me what the heck is going on with this? You can see for yourself at the end of the video linked below. (My kids and I are documenting or progress and/or lack thereof on a YouTube channel.) Thanks, all! Michael