260z round top conversion: a couple questions

[Captain Obvious 2,237]

Yeah, I would keep the AB valve if you can. You could slap a little cone filter on the fresh air end just to keep the bugs and birds out. The ones that a lot of people put on their valve cover nipple? Something like that?

And yes, the 260 balance tube is much more complicated than the 240 version. If you're positive that you won't be running EGR then you can make things look a lot less basket of snake-ish by switching over.

[Cooper260z 9]

Will the valve work if there’s just a filter on it? I thought that it would need vacuum or something?

[Cooper260z 9]

Also, is the linkage for the 240z different then the 260z? I don’t have a good photo to show what I’m talking about at the moment, but on the balance there are two ears that a piece of linkage goes through, and on my l24 balance tube those two ears are much closer together so I’m not sure if my linkage will work with it 

[dutchzcarguy 208]

8 hours ago, Cooper260z said:

after market air filters I bought didn’t come with one of those plastic elbow ports that they usually come with

No idea what your talking about..

8 hours ago, Cooper260z said:

remove the anti backfire valve or should it be kept at all costs

Keep it, it's there for good reason..

[Captain Obvious 2,237]

9 hours ago, Cooper260z said:

Also, is the linkage for the 240z different then the 260z? I don’t have a good photo to show what I’m talking about at the moment, but on the balance there are two ears that a piece of linkage goes through, and on my l24 balance tube those two ears are much closer together so I’m not sure if my linkage will work with it 

Yes, the linkages are different. Remember back on page 1:

On ‎9‎/‎11‎/‎2020 at 9:08 PM, Captain Obvious said:

And one tricky bit that you haven't hit yet will be the throttle linkage... The piece between the flat tops is shorter than the one used between the round tops. So for that reason, when you replace the flats with rounds, you'll find that center linkage piece to be a little bit too short. So keep your eyes open for some round top linkage pieces.

And about the anti-backfire valve... It's got three connections:

1 - High vacuum from the balance tube side
2 - No vacuum from the air cleaner side
3 - Control line to open the valve when the intake manifold vacuum goes high enough

The connection that you put the little air filter on is the no vacuum fresh air side. The other two ports connect to vacuum sources. Does that make sense?

[Cooper260z 9]

Okay, I thought the only different linkage was the one in between the carbs. 
And yes that makes perfect sense in regards to the AB valve, thank you. 
 

I’ll get back after it then and see what stumps me next!

[Cooper260z 9]

Seems like it hasn’t been that long but I swear I’ve made progress since the last post. 
 

My l24 heat shield is running into the crankcase vent, preventing it from lining up with the mounting holes. Is there a popular solution other then cutting the shield? Because that’s what I’m seeing as an obvious “fix” I don’t have a problem with cutting it but if I don’t have to then then it would be nice to keep it whole 

[zKars 1,200]

Does your crank case vent tube look like #1 or #2

 

L28 style

L24 style. Not sure if L26 came with #1 oe #2... I can see how #1 would be in the way of the heat shield

Edited October 18 by zKars

[Cooper260z 9]

It’s like #1 I think. I was too motivated to wait any longer after lunch so I went ahead and cut it. It didn’t take much

[Cooper260z 9]

The carbs are on and most things are hooked up but I’m having a pretty sizable issue. 
The car will only run at 2000/4000 rpm and 200 rpm... nowhere in between 

 

When it’s running between 2-4000 it runs well and throttle response is crisp. 
When it runs at 200 rpm, the throttle mostly just chokes the system. 
Also the engine will seemingly just randomly shoot up from 200 to 2000 rpm. 
 

These carbs could be bad and in need of a proper rebuild, that’s entirely possible, but I really don’t much about the SUs so I’m not sure and I dont have the budget for the rebuild right now so I’m seeing if this problem can be fixed.

 

Here is what I’ve done in an attempt to fix the issue as well as notes on the situation: 

I’ve moved the mixture nuts around and that seems to be a good way to get it to idle at 2000+ (turning the nut to richen the mix) but it can also jump to high rpm when the nuts are only around three turn out. 
The choke cables are currently disconnected because I found that I need a piece to affix the cable to these carbs, but I can manually manipulate the choke to get it to idle a bit better but still not great.

The pistons in the carbs move up and down as I’ve read they should, with the 20wt oil they’re a bit harder to push up and slowly fall back to the bottom.

One of the needles was a bit bent but I was able to straighten a bit by using the technique in the ztherapy video.

I do have at least one confirmed vacuum leak and am not ruling out that I could have more. 

I’m guessing the sudden jump in rpm is one or both pistons rapidly going up?

Im also getting a lot of backfires out of the carbs when it’s running at low rpm

And here’s a picture of what the engine bay currently looks like, still very much a work in progress I know things aren’t prefect but please point out anything that could be an immediate issue 

 

Im actively researching trying to find information on a situation like this but I figure while I search might as well see if anyone can help me out. Thanks again to everyone that’s helped so far!

 

Edited October 28 by Cooper260z

[Captain Obvious 2,237]

Did the car run reasonably well with the flat tops on it? First thing would be to make sure it's a decent running motor in the first place.

Start by getting rid of the vacuum leaks. Just to test things, you can cap off the entire anti-backfire system. You can also pull and plate off the EGR just in case it is sticking open a little.

Other things maybe? You've got a rubber vacuum line connected to the front manifold right behind the carb. This is the line that splits and goes to your throttle opener and AB valve. Well there's another nipple (broken off) right next to that one. What's up with that? Is that a leak?

And the throttle linkage doesn't look quite right to me. The stop up by the EGR valve is nowhere near making contact. And the connection nut on the rear carb looks like it's bent and/or loose? Is that an attempt to mitigate the "too short" linkage piece between the two carbs?

[Cooper260z 9]

Yes the second vacuum port on the manifold broke, I tried filling the hole with epoxy to seal it but that’s where we’ve found a vacuum leak when spraying brake clean in the area. 
 

I noticed that the linkage isn’t going all the way “home” I’ll say, and isn’t lining up with the service idle adjustment screw but I haven’t figured out why yet. I hadn’t noticed that connection but being bent before, it does look a bit tweaked, I’ll check that out. But this is the correct (I’m pretty sure) linkage for a 240z. 
 

With the flat tops the car did run quite well. I just noticed a few hiccups that I decided we’re likely carburetor based which is part of the reason why I was making the switch. So yes, using how the engine ran with the flat tops as a baseline, it should idle and drive fine. Maybe the tweaked throttle linkage is the main issue, I suppose that could explain the high idle but not the sudden idle jump and climb I don’t think. Also doesn’t explain the extremely low idle I don’t think 

[Cooper260z 9]

Alright, so this piece of linkage that connects to this bar is what was keeping the linkage from returning all the way. I’m not sure what this mechanism is for yet but clearly these linkages aren’t completely compatible with each other. 
Also I’m not sure exactly what order these flats are supposed to go in, maybe that’s my issue. 
 

But disconnecting that linkage fixed it! So I guess the reason that it was idling at sich high rpm was simply because the throttle was being held open... who woulda guessed. So with that detached the car will idle and be responsive so I can try to dial the carbs in and fix some other issues I can already see!

[Mark Maras 1,071]

 Concentrate on the float levels (primary mixture adjustment) and balancing the carbs first.

[Cooper260z 9]

Thank you @Mark Maras I will after I plug the air injection inlet so that I can concentrate on something other then the large exhaust leak in front of me. 
And while I wait on that, I’m looking at the linkage to this diaphragm. (That wasn’t meant to sound snarky or rude, if it did I apologize)
 

So, I got the linkage from the l24 which I now see is significantly longer. Is this the proper configuration? Suspending the adjustment screw on the front carburetor?

[Captain Obvious 2,237]

That vacuum servo that is giving you troubles is called the "throttle opener", and it's job is to hold the carbs open a tiny bit under very high intake manifold vacuum conditions. Essentially it is supposed to prevent the manifold vacuum from going above a certain level. It's in the same category as the anti-backfire valve.

And to answer the question about the linkage... No, that adjustment screw is not supposed to be floating in air a half inch away from the linkage. I assume that it's all caused by incompatibilities between the 260 bits on the balance tube and the 240 bits below.

Last answer... Yes, that is a round top style piece between the two carbs.

So for now, you can just leave the throttle opener disconnected and get the car running well. Then once you're over the  basic hurdle, you can get yourself a round top balance tube and throttle opener maybe.

[Cooper260z 9]

if I can get a hold of the 240z linkage then I’ll use that balance tube, but I do have the 240z throttle opener if it would make a difference but at face value they seem the same.
I feel like a bit of a bother at this point but do you know the correct way to attach the throttle opener linkage? The only way I see to attach it leaves that adjustment screw linkage piece suspended independently. Or are you saying that there isn’t really a way to do it because of my amalgamation of parts going on here? 

[Captain Obvious 2,237]

I'm saying that I don't remember the exact locations of all the parts involved and it's possible that the throttle opener may be mounted lower on the 240 balance tube. If that's the case, the throttle opener device itself may be identical between 240 and 260, but they may have put it in a different spot between the two.

Same goes for the throttle stop you're hovering above on the balance tube. That little stub of throttle linkage may be significantly different between the two. I just don't remember.

I know lots of people have done flat top to round top conversions and have overcome these hurdles and I bet there are pictures out there on this forum that would answer some of those questions.