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Carburetor Conundrum (260Z)


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Well THERE'S your problem!! Yes, that nail was holding your butterfly open and not allowing it to close completely.

Some answers to additional questions:

That hole is intended to be a ported vacuum source to drive the distributor advance. There is supposed to be a vacuum tube nipple pressed into the carb body that allows you to connect a rubber tube between the front carb and the distributor advance diaphragm. It appears someone sheared the nipple off and decided to plug that hole with a nail instead.

And about the "insert grease" engravings... It appears that someone drilled little holes in the carb mounting face with the intent of using them as lubrication grease ports to lube the throttle shafts. Probably a waste of time, but since they are sealed off by the gasket between the intake manifolds and the carb faces, it probably doesn't cause any harm either.

So for both of those items... THAT'S something I've not seen before!!  LOL

Bottom line... Plug that vacuum nipple with something that does not stick down into the carb throat and I bet your idle issue will be pretty much taken care of.

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Posted (edited)
14 hours ago, Captain Obvious said:

Bottom line... Plug that vacuum nipple with something that does not stick down into the carb throat and I bet your idle issue will be pretty much taken care of.

That’s the plan now… JFC I was losing my mind. I know I should have just taken the damn thing off earlier but it always feels like opening a can of worms to me. You would think I’d be used to it by now lol…

It does have a vacuum advance hose going to the distributor, it’s just attached a little farther back, behind the spacer on the manifold. I guess it’s a by-product of mixing and matching parts off different years of car.

Many thanks again to everyone who has chimed in on this, here’s hoping the boys and I can start working on something else soon! lol 

Edited by DadAndLadZ
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8 hours ago, DadAndLadZ said:

It does have a vacuum advance hose going to the distributor, it’s just attached a little farther back, behind the spacer on the manifold. I guess it’s a by-product of mixing and matching parts off different years of car.

Well that vacuum connection you are using now is full manifold vacuum, but the system was designed to use a ported source that peaked advance at light cruise throttle (slightly above idle). With your direct manifold vacuum source, you will have peak advance at idle, and it will drop from there. I'm no timing expert, but I'm not sure that's what you want.

And I don't think your vacuum connection is really a remnant of mixing and matching parts from different years. I think it's a remnant of snapping off the original ported vacuum nipple and then looking for some place else to connect that vacuum line.     ROFL

Either way... So when will we get an idle report to see if taking that nail out allows the front carb to operate properly?

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1 minute ago, Captain Obvious said:

Either way... So when will we get an idle report to see if taking that nail out allows the front carb to operate properly?

Wow, good info, thanks! We will put it back together ASAP!

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11 minutes ago, Captain Obvious said:

I have very little doubt that things will be a whole lot better now, but after four pages, I just want to hear it from you to be sure!   :beer:

Oh you guys will hear for sure! lol … THANKS most sincerely for your patient help!!!

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Yeah, I think I would recommend running a return line, but that (like a lot of other details) can wait. I just want it to idle correctly for now!  Haha!

I'm no regulator expert, but I'm not a big fan of dead-headed systems in carb applications. This non-expert would prefer a bypass regulator system with a return line.

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Oh, and DadAndLadZ, forgot to mention.... Now that you have that nail pulled out and all the idle screws backed completely out, both carbs should be shutting tight-tight. And if that's the case, it probably won't even idle. You might find that you have to manually hold the throttle open a little bit or screw the idle screws back in some just to get it to run at all.

Or you could pull the choke lever. As you saw, it should open the throttle plates a small amount.

Pretty sure you would have figured that out, but just throwing that out there.

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Well, we put the carbs back on today and did a rough dial-in and even managed to take Project Z out for a little sprint around the block. 

I have been thinking about what @Captain Obviouswrote the other day regarding where the distributor advance hose goes, and found some pix of SU carbs where the "nail hole" did indeed have a hose nipple on the exterior. I still have the advance hose hooked to the port right behind the carburetor in the manifold for now but I totally see what the Cap'n was talking about. Would something like this (image below) work, if I tapped the hole out a little to thread it in?

image.png

Or is there a readymade part for this? Any advice on this is welcomed.

Oh, and I put together a new video for our YouTube channel, too. This one was more fun than the last couple because it felt like we actually made a little progress!

 

Thanks again, folks. Let me know what you think about that vacuum port biz above, if you have an opinion on it. Oh, and also on the overheating! Plz/thx.

 

Best,
MC in Wichita

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Well that sounds like significant progress. I skipped through the video really fast though. I'm not much on videos, especially long ones. Maybe it's just me, but I'd just rather read a couple sentences about it instead of having to watch a 15 minute video about it. LOL  A text synopsis about the video would be better for me.

"We removed the nail. Put the carbs back on the car and the idle was down where it was supposed to be. Now that the carbs were behaving like they should be, we did a quick tune and set the idle speed and sync of the carbs using a uni-sync tool. After that, it was running well enough that we actually took it out on the road for a quick road test. Did great, all things considering."

That would be better for me.  ROFL

So about that ported vacuum source for your distributor advance... I'm not sure what you have is salvageable, at least not without extraordinary means. The problem is that the hole into your carb throat is way bigger than it should be. Looks like when they snapped off the vacuum nipple, they drilled out the remains and went too deep. Like all the way into the carb throat deep.

In other words... Some previous owner drilled through your carb body and ruined it. The vacuum hole is supposed to be small. Thirty thousandths maybe? And the one in your carb looks like it's an eighth inch or larger. The bottom line is that both the size of that hole and the location are critical and yours has been wallowed out to three or four times it's normal size.

So if you want to run ported vacuum, you'll need a new front carb. Not a lot of fun.

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It may be that the current vacuum advance from the intake manifold is okay. Should be able to tell how the advance is working that way with a timing light. If it's working okay this way the hole in the carb could just be plugged (with a nail 😁, not really).

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Just now, w3wilkes said:

It may be that the current vacuum advance from the intake manifold is okay. Should be able to tell how the advance is working that way with a timing light. If it's working okay this way the hole in the carb could just be plugged (with a nail 😁, not really).

I do have a timing light and we will be working on that coming up.

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Here's a couple pics of the 72 carbs (which is what you have in your car) and the vacuum nipple:
P1200703.JPG

And here's the port on the inside. Note that I'm holding the butterfly open a little bit so you can see the hole. With the butterfly all the way closed, the hole is completely covered. Also note how small the hole should be:
P1200705.JPG

 

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Glad to help.

So for the vacuum advance thing... The ported vacuum will have a sharp narrow peak just above idle. At idle, there will be very little advance, and above light cruise, there will also be very little advance.

In contrast, the direct manifold vacuum will be highest at idle and gradually drop from there to zero advance at WOT. The profile is very different than the ported source.

That said, there are plenty of people who think that direct manifold vacuum is actually better, and plenty of other people who think that all the vacuum advance can be removed completely and just set the base timing higher instead.

Here's a discussion we had a long time ago about such things. Quickly went over my head:
https://www.classiczcars.com/forums/topic/41935-ignition-timing-theory-port-source-vs-manifold-source/#comment-377416

 

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On 7/7/2022 at 1:13 PM, SteveJ said:

By the way, if you haven't done so already, go to https://www.classiczcars.com/files/ and download the factory service manual for the 260Z. Yes, your car has been modified, but the FSM will help you identify how it looked from the factory.

I would also suggest getting the FSM for an earlier Z, say the 72, to compare the setup for the earlier round top carbs.

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On 7/11/2022 at 12:02 PM, DadAndLadZ said:This is the one real drawback to these sidedraft units…you can’t see down em! Much obliged.

If you need to look into the throttle bores, a mirror will work. A light will make it even easier to see.

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On 7/16/2022 at 7:34 PM, DadAndLadZ said:

Just using some motor oil, I think it’s like 10W40. As far as I can tell it’s staying in there, I haven’t seen it leaking anywhere. 

I believe the FSM. Calls for 10wt non detergent motor oil.

I use ATF, and usually only a few drops.

 

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 I remembered 20wt. being used but the oil just slows the piston rise during acceleration. With the butterfly open and the piston slowly rising the air speed thru the venturi speeds up drawing more fuel from the bowl.

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OK, I just read through the entire nail in the vacuum port saga, and would like to offer some observations.

In the first post there is a video, and you have your son start the car. You direct him to pump the accelerator twice before cranking the engine. This is a common misconception carried over from engines equipped with carburetors with automatic chokes and accelerator pump circuits. Stepping on the pedal before starting does two things. First, it allows the choke plates to fall closed. Second, it squirts a bit of fuel into the intake passage.

This does nothing with the SU carburetor, as it has no accelerator pump circuit and no fuel will be introduced prior to startup. Also, the SU carbs don’t have a choke plate, the “choke” lever simply opens the throttle slightly to facilitate more air and extra fuel (only added as the pistons draw air through the carbs into the engine, which also draws fuel from the float chamber through the orifice where the tapered needle is in the carb throat).

I also didn’t see and mention to check for vacuum leaks (although Mark Maras did mention that excess air, not fuel, will cause high idle). 
 

Ultimately the culprit was found to be a hack job by the previous owner who did the carb conversion.

 

My .02 cents.

 

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