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Cooper260z

260z round top conversion: a couple questions

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I know this topic has been heavily discussed in the past but I am unclear on a couple things and am looking for some help.

to set the scene, I’m working on an early 74 260z that to me looks like it’s bone stock and has almost everything in the engine bay that it came with. I have two sets of round top carbs and a mostly complete l24 sitting on a dolly. Also this my first Z car and I’m not too familiar with them yet

Neither of the round tops that I have, have the holes on the manifold side of them that I think I are used to circulate water? So if I want to use these carbs should I use the manifold that’s on the l24 that also does not have the holes to circulate water, as opposed to the manifolds that are currently on the car? The l24 manifolds do have a water line going between them though, Incase that is relevant. As a side note, the round tops did not come on the l24.

If I use the manifolds that are in the l24 it seems to makes sense to also use the balance tube that lacks the provisions for the egr diaphragm and such. But if I use the l24 balance tube then what do I do with the two hard lines that are coming out of the balance tube that are currently on the car? Those two hard lines seemingly being used for water and egr. The l24 balance tube has one port at the end of it but that port is currently blocked off and I don’t know what would’ve been there originally, if it would’ve been water or gone to the exhaust manifold like the 260 one.

And on the thermostat housing on the 260, a port is blocked off and I’m wondering what that should go to.

I’ll add some pictures of what I’m referring to and am happy to take more pictures if necessary.

I’m trying to get at least most of the details of this conversion sorted out before taking the car apart, because even though it’s not registered and I’m not driving it around, the car does run and drive and occasionally needs to be moved and that’s obviously an easier task when the car runs.

So if my questions aren’t too annoying, I’d appreciate any and all help anyone has for me. Thanks!

The pictures in order of appearance show the blocked off thermostat housing port, the hard lines on the 260 balance tube and the single blocked off port at the end of the l24 balance tube 

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Oh, and if I take the intake manifolds off I assume I should change the manifold gasket, which would mean also taking the exhaust off. Is that correct?

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Let's do this a little bit at a time. First, lets talk about intake manifolds...

In one year, and in one year only (1972), they passed water between the intake manifolds and the carb bodies. So the 72 manifolds have water holes in the face where the carb mounts. None of the other years have those holes. Earlier round top manifolds don't have those holes, and later flat top manifolds don't have those holes. So as long as you don't have holes in the surface where the carbs mount, you can mix and match any carb with any manifold.

Does that make sense? It sounds like none of the intake manifolds you have holes where the carbs mount, right?

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@Captain Obvious okay, That makes sense. Id recently seen a flat top carb by itself and saw that it had various holes in it, but I haven’t taken them off my car yet so I wasn’t sure if the manifold had water holes. 
 

The carburetor insulators I see for sale on zcardepot all have holes at the bottom, but I assume I can just ignore those and use them?

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If you're planning to re-use your original 74 intake manifolds (and I see no reason not to), you will not have holes on the carb mounting face. With that in mind... Yes, you can use any of the carb insulators, including your original 74 insulators.

There were three versions of those insulator / spacers:

1) 70 and 71 uses insulators that are square-ish and thick.
2) 72 insulators were thick like the previous years, but they are no longer square. Now they are taller than they are wide. Also, they have holes in them (for the water passageways between manifolds and carbs).
3) 73 and 74 spacers were the same shape as 72 (taller than they are wide), but they are thinner and don't have water holes at the bottom.

The only question is do you want thick or thin? The reason they went to the thinner insulators in 73 was because they were running out of room between the air cleaner and the side wall of the engine compartment. If you've already got a set of the thin ones there, you could just run them. If for some reason you want to replace them with a thicker version, then you could do that too.

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Wow, are forums not the best aftermarket car part there is? Thank you very much @Captain Obvious that clears up a bunch right there. Although I’m sure I’ll come up with a few more questions next time I look at the car. 
 

But what about that blocked off thermostat port? The previous owner told me that he or the owner before him has “blocked off the water to the intake” but I can see that the water still goes through the intake and even leaks onto the exhaust I think. So what should that blocked off port go to? Incase I should put it back. 

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On the flat top car models (73 and 74) there are actually three paths for coolant to flow through the intake track. All three of these paths start at the thermostat housing.

First path comes out of the thermostat housing and passes through the intake manifolds.
Second path comes out of the thermostat housing and passes under the front carb and then up into the balance tube (near the EGR valve).
Third path comes out of the thermostat housing and passes through the carburetor bodies.

All of those paths then gather back together again at the back corner of the block and from there they flow through the hard line around the back of the block.

So I can't tell exactly which of those three paths is the one capped off, but you should be able to trace the lines knowing what you're looking for. If not, take some other pics from different angles and that should help with the ID.

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Alright then, the blocked off port would seem to be the one that goes to the carburetors then. 
 

Thank you so much @Captain Obvious I think you’ve cleared most of questions! 
So I think I can start swapping the carbs then once I get new gaskets 

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Glad to help and good luck with the project.

We haven't talked about balance tubes at all, but the bottom line is you can use any of them, including the one already on the car. The only "down side" to that one is the complexity and for that reason many people doing the round top conversion replace them with something simpler.

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.

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I figured I’d leave the balance tube for now and go back for that later. But if you have some insight on what I should with those hard lines when I do want to change the balance tube, then I’d be happy to hear it! 
 

I do have the round top linkage, I think. The guy I bought the carbs from, gave me two different sized center linkages with the confidence that one of them would be the right one. So hopefully one is. 

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I think the plan of leaving the manifolds and balance tube is a good one. If everything works out well you can always swap out the balance tube later if you get tired of looking at the EGR parts. As for the hard lines, some of them are water and some of them are exhaust. When/If you're going to switch over to a 70-72 balance tube, just cap all the water and exhaust ports off. If there's something specific you're concerned about, post a pic and we can talk about it.

And it sounds like you're fine with the linkages. I think they only made two lengths (longer for the round tops and a shorter version for the flat tops), so if you got two different lengths from the guy you bought everything from, it sounds like you've got what you need there.

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I think you also need to consider the vacuum line for the power brake booster.  It's a preformed rubber hose and the routing is slightly different for a 73/74 than an older balance tube.   I have a 73 with round tops.  My PO kept the original intake and balance tube, but I have a really nice polished set of intakes and matching balance tube that I want to swap on there this winter.  However, I'll have to buy a different vacuum line.  

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noted. Thank you guys so much. I don’t know why this change of carburetors seemed so daunting before. Guess I just got psyched out by reading about the problems people ran into. 
 

If there’s anything you think I should know about this, I’d love to hear it!

Thanks again!

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On ‎9‎/‎9‎/‎2020 at 10:25 PM, Cooper260z said:

But what about that blocked off thermostat port?

You are in truly GREAT hands using the Captain's explanations and advice, but I will chirp in here with some comment.

This is all dependent on a "given" that you intend to convert to round top carbs from the flat tops.

The lower thermostat housing that you have with the blocked off port is interchangeable in mounting with the earlier lower thermostat housing. The upper housing caps are the same. You could swap out the lower housing, install an earlier manifold water fitting and reduce some of the clutter and previous owner damage. When and if you decide to change over to the earlier balance tube you will also have the opportunity to change to the earlier type water piping at the rear of the engine block. This would eliminate capping off the now un-necessary water lines on the water piping used on the later flat tops in that area.

The routing of the water lines with the "Y" connection shown in your lower photo is on the wrong side of the block/ valve cover.  It should be on the opposite side ---mounted below the spark plugs. It reduces in diameter and converts to a hard metal pipe the wraps behind the engine and connects to the rubber coupling shown above the heat shield on the lower photo.

If you have on hand all of the parts shown in your photos you have most everything you need to make a complete proper conversion. I don't know if you have on hand the earlier metal water pipe that wraps around the rear of the engine.

On ‎9‎/‎8‎/‎2020 at 8:53 PM, Cooper260z said:

The l24 balance tube has one port at the end of it but that port is currently blocked off and I don’t know what would’ve been there originally, if it would’ve been water or gone to the exhaust manifold like the 260 one.

The capped port on the balance tube shown in your third photo should be plugged just like it is on the very end. Just a plug, instead of a fitting with rubber hose and a bolt. The remaining  fitting is where you obtain vacuum for your brake booster and can be left there (vertically) or moved to the end (horizontally) depending on which hose type you have coming from the brake booster. 

I did the complete round top conversion on my 73, but then later made the decision to return to the "original" flat tops with all the associated air/ water/ vacuum plumbing in going back to a "stock" configuration.

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Is this the pipe @Zup?

Not sure about my position for the pic but I'm pretty sure this is the one off my '72. I deleted the coolant flow through mine so I would be willing to provide this to a good cause. I.E, if it will be used?

Cliff

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Okay, walked my lazy arse downstairs and it is the pipe off a '72.

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Edited by siteunseen
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@Zup I do have the other thermostat housing and early water line. The water pipe you see in the bottom picture is just an extra one that’s sitting on the engine. The guy I got the engine from just threw that in there and I don’t usually say no to free parts. That whole l24 was free. 
 

So then to change the balance tube I’d just have to figure out how to block the hole in the exhaust manifold I think. 

 

Also I’m planning on to use the heat shield from the l24 because it has the hooks for the rerun springs for the round tops which I think my current heat shield lacks 

Edited by Cooper260z

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Right on Site!!

 

Coop----sounds like this is under your control now. Best of luck with your conversion!

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5 hours ago, Cooper260z said:

I’d just have to figure out how to block the hole in the exhaust manifold I think. 

The MSA 6 to 1 ceramic coated headers do a pretty good job of that. ?

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On 9/13/2020 at 8:51 PM, w3wilkes said:

The MSA 6 to 1 ceramic coated headers do a pretty good job of that. ?

The thought had crossed my mind. Might do

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I’ve finally started the process of taking things off to put the round tops on, and I have another question that I don’t THINK I’ve asked yet. 

What should I do with this diaphragm? Not exactly sure what it does but from what I can tell, earlier 240Zs at least don’t seem to have them so I’m guessing it isn’t necessary and I’m not sure if the aftermarket air filters I bought (but haven’t received yet) will allow for it to plug in. 
 

So really the question is do I add that port in the balance tube to the list of things to plug? And also what does that diaphragm do, just so I have that knowledge. 
 

Also what is that vacuum hose for and can I plug that? Because I’m not sure if the new air filters will have a port for that either 

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That diaphragm device is your anti-backfire valve. The round top cars used something similar. Not located in the exact same spot, but pretty close.

As for the vacuum line, that went to your "idle compensator" device built into the air cleaner. It's job was to lean out the idle mixture a little bit when the engine got hot. The round top cars didn't have anything like that and you can just pull the hose off the balance tube and put a cap over the nipple.

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On 9/13/2020 at 6:10 AM, Cooper260z said:

If there’s anything you think I should know

I wanted to ad.. why all those gardenhose clamps, get yourself some good (original) clamps.  they are much better. Those are used in planes for good reason.

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thanks again!

 

@dutchzcarguy I thought the same thing! That’s how I got the car, they’re are so many I thought that they might've been Original. I change them out every time I take one off  

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Should I be keeping the anti backfire valve? I’m guessing it was on the 240z and they kept onto the 260z then maybe it served a good purpose. My problem though is the after market air filters I bought didn’t come with one of those plastic elbow ports that they usually come with so I’m not sure how to hook it up. I plan on getting a 240 air filter housing but for the time being, can I remove the anti backfire valve or should it be kept at all costs?

 

Also as things are being learned, it’s starting to make more sense to change to the 240 balance tube also, for simplicity sake 

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