# 1968 Vacuum Lines and Carburetor



## knucklehead12 (Nov 14, 2013)

I recently purchased a 1968 GTO Convertible. Just going through everything to get it road safe and need some help. On the manifold, there is a vacuum line tree with 5 lines coming off it. The bottom line was broken off and I do not know where it goes. It is on the side with 3 lines toward the passenger side. Also, there is a vacuum line in the center of the manifold in front of the carburetor that has a screw in the hose dead heading it. Where is that supposed to be connected? And lastly, when I mash the gas pedal, there is a hesitation before the engine accelerates. What causes that? Any help would be greatly appreciated.


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## Matthew (Feb 11, 2011)

The vacuum tree you are talking about is a Thermostatic Vacuum Control Switch (TVS). Your broken off line is likely causing a vacuum leak. This maybe causing your engine performance problem. There's normally a rubber switch plug with vacuum lines connected that mounts to the 5 ports. Is the port broken off the TVS switch or is it one of the fittings has pulled out of the rubber switch plug? If the plastic port has broken off the TVS, you will need to replace the unit. 

The manifold port you are talking about supplies vacuum to the Thermal Vacuum Sensor that would be in the original air cleaner. I assume you do not have the original air cleaner and the line is terminated. Matt


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## knucklehead12 (Nov 14, 2013)

Matt, thank you for the information. I fixed the vacuum line from the manifold to the air cleaner. The last one in question is on the TVS on the side with 2 lines (I mistated this in my original post). I replaced all the vacuum lines that were intact, but the bottom line on the TVS on the side with 2 lines was broken off and I cannot figure out where that line goes.
Lee


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## Matthew (Feb 11, 2011)

Lee, ha, not so easy to answer this question, and the answer might be, it goes no where or it is not needed. First, do you have manual or automatic trans, manual or power brakes, and hideaway headlights? To achieve the original set up of vacuum lines, you will need to check for: 1) one or two port vacuum advance on your distributor, 2) distributor vacuum control valve, and 3) original era quadrajet model 702826x (x meaning another number). If you do not have all three of these items, the answer is likely it is not needed. How confusing is that? Matt


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## Matthew (Feb 11, 2011)

Lee, I failed to mention that the port being broken off the TVS will create a vacuum leak for any set up you have. It will have to be plugged/capped if it is not needed. Replace the TVS. Matt


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## knucklehead12 (Nov 14, 2013)

Let me check on all this and I will get back tonight.
Thank you
Lee


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## jmt455 (Mar 26, 2012)

Found these images several years ago when I was building my first 68 GTO.
They help identify the routing of the TVS hose harness.

IIRC, none of the TVS hoses go to the air cleaner vacuum motor or sensor. The vacuum line for the air cleaner "Thermac" motor is the hose that is leaning against the front of the carburetor in the photos.


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## Matthew (Feb 11, 2011)

Great pictures from JMT. Just a thought. These pictures may or may not be right for your car. As an example, if you have a manual trans and a two port vacuum advance there would be one line running to the distributor from the TVS which you see in the lower portion of the second picture, and another coming off the distributor vacuum control valve (back right corner of the intake). Also not apparent in these pictures is a line for hideaway headlights - if you have them. The set up in these pictures appear to be for a power brake car as it looks like there's a larger port at the rear of the carb for the line to the brake booster. If you have manual brakes, you would not see the tee connection, but rather an elbow... all that said, disregard most of this if you don't have a quadrajet.  Matt


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## knucklehead12 (Nov 14, 2013)

Thank you for the pictures, That helps a lot. I do have a manual transmission but no hideaway headlights. I do have the original quadrajet. Kids took me away from working last night. Hope to get back on it tonight.
Thanks
Lee


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## knucklehead12 (Nov 14, 2013)

I finally figured out the problem. I do not have the distributor vacuum control valve that sits on the back passenger corner of the manifold. I see that they are about $100, so my question is, is there a work around, or do I need to just break down and buy one?
Lee


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## Matthew (Feb 11, 2011)

Lee, that depends on whether or not you have a two port vacuum advance on your distributor. If you have only one port on the vacuum advance, then no. And, I do not believe anyone is producing one right now. If you have the original distributor that had two ports, and you want the original set up, then yes. Do you have to have it if you are not concerned about originality, which it sounds like you are not, then no. Knowing what these components do is kind of important.

Distributor Vacuum Control Valve. This is used on all manual transmission applications and controls the type of vacuum going to the vacuum advance. During deceleration, this valve changes from ported vacuum to manifold vacuum allowing maximum advance. It is part of emissions control.

Thermostatic Vacuum Control Switch (TVS). In all applications, the TVS serves as a safety device to help prevent overheating. The switch is located at the front of the intake manifold, threaded into the coolant passage. There are positions inside the TVS related to coolant temperature. Standard vacuum flow is ported vacuum from the carburetor to the distributor vacuum advance. Whenever the engine coolant temperature rises above 230 degrees F, the TVS switches the distributor advance from ported to full manifold vacuum. This advances the timing about 20 degrees at idle, allowing the engine to cool down to normal operating temperature. After the engine cools, the system returns to ported vacuum.

I have read where some folks have eliminated the TVS. I'm not an expert, but it sounds kind of important to me. Going back to your original issue, you said you have one plastic connector broken off the TVS. I can only assume this is creating a vacuum leak. I would recommend replacing the TVS, and if that port is not needed, which it sounds like it is not, then cap it off. Hope none of this is too confusing. Matt


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## knucklehead12 (Nov 14, 2013)

Matt, Thanks for the information. That confirms what I had read. As far as the TVS goes, I may have mislead you, what I meant to say is that the hose was broken off, not the TVS, and that is why I could not figure out where the other end was connected to. Obviously it was not connected and must have been just open. I will get a distributor control valve and see if that fixes my flat spot acceleration issues.
May take a little time as I need to find another sack of cash to buy that. Will let you know.
Thanks for all your help,
Lee


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## Matthew (Feb 11, 2011)

Lee, gotcha on the TVS. As for the distributor vacuum control valve... in my opinion, it will not effect your hesitation problem. Matt


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## knucklehead12 (Nov 14, 2013)

I understand, will let you know. I was hoping it was a simple fix, but I figured I was going to have to rebuild the Quadrajet to fix my problem. Don't know if you know, but I do all my work myself, is it that hard to rebuild a Quadrajet, or should I send it off to a pro?
Lee


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## Matthew (Feb 11, 2011)

Lee, is it that hard to rebuild a Quadrajet? Well... not really. Sounds like it might be your first. You will want to do some research before taking that little fellow apart. Your fix maybe as simple as an accelerator pump. Some will recommend a professional like Cliff Ruggles (1 year waiting line). My first was in like 1975 and I was supervised by a guy that had done a bunch. Matt


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## knucklehead12 (Nov 14, 2013)

Might buy Cliff's book and give it a shot. Thanks again.


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