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I have a 74 260z with an L28 that has 240z carbs retrofitted on it. Its a total frankensteins monster and I love it. The only problem is that I'm 17 years old and I don't have a dad to help me fix all the problems with it, so I am going to have to rely on all of the other dads here on the forums lol.
The problem happened right after I sent the rear carb to get repaired; it came back in perfect shape but the dashpot was obviously empty, so I filled it with the recommended 50-20 motor oil. Then upon installing brand new spark plugs, the car fires right up and skyrockets to full throttle indefinitely. I checked the idle screw, it was almost all the way unscrewed. The throttle butterflies were closed on both carbs so they weren't stuck open either.
To test if it was the newly refurbished carb, I took away the fuel and spark from the back 3 pistons and respective carb and let the engine run on the front 3 cylinders for a few moments to check and sure enough, the engine spit and sputtered at its normal low RPM. That leaves the rear, refurbished carb to be the problem.
Do you guys have any ideas as to what could cause this? I suspect maybe a vacuum leak has something to do with it but I'm not entirely sure.
 

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You have a vacuum leak probably in the area between where the carburetor mates with the intake manifold. Do the hand-pump cigarette smoke test outlined many times on this forum. That will show you where your vacuum leak is.

Are you using an old gasket between the carbs and intake manifold?
 

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Discussion Starter #3
You have a vacuum leak probably in the area between where the carburetor mates with the intake manifold. Do the hand-pump cigarette smoke test outlined many times on this forum. That will show you where your vacuum leak is.

Are you using an old gasket between the carbs and intake manifold?
I think I am. I have a roll of gasket material that I could make my own gasket, but should I buy a new one or is making my own a possibility.
 

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I think I am. I have a roll of gasket material that I could make my own gasket, but should I buy a new one or is making my own a possibility.
Just go buy one they really aren't that much money and cutting a custom one is a hassle. Before you go do that though, smoke the engine to see if you even have a vacuum leak. No sense in buying a new gasket and taking the carbs off if you don't have a leak.
 

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Discussion Starter #5 (Edited)
Just go buy one they really aren't that much money and cutting a custom one is a hassle. Before you go do that though, smoke the engine to see if you even have a vacuum leak. No sense in buying a new gasket and taking the carbs off if you don't have a leak.
I was already in the process of making a new gasket and removing the carbs when you responded. I made all new gaskets the come after the insulator blocks and I replaced the damping oil in the dashpots but absolutely nothing changed. I got a video of the engine starting and the pistons in the carbs lift up as if they are being throttled, but you can easily see that each throttle on the carbs isn't moving at all. The only thing I can do now is try to get a video looking straight down the carbs at the butterflies in the back to see if they're moving or not because that would indicate where the air is coming from. I don't have an cigarettes or cigars so I'm not able to do the smoke test easily and I'm still not sure entirely how to do it.

Heres a video of the of it in action (Fixed the link)
 

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Sounds to me like you have the balance screw turned all the way in possibly. There is also an adjustment for the throttle closer device (vacuum actuated dashpot and linkage on top of the intake manifold balance tube). That can be overadjusted and cause it to overrev. If you hook up the vacuum line to it incorrectly it will wind right up.
 

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Discussion Starter #7
Sounds to me like you have the balance screw turned all the way in possibly. There is also an adjustment for the throttle closer device (vacuum actuated dashpot and linkage on top of the intake manifold balance tube). That can be overadjusted and cause it to overrev. If you hook up the vacuum line to it incorrectly it will wind right up.
There are no vacuum lines connected to my carbs, that's how they were when I got them and it ran just fine without them. I thought that most, if not all, of the vacuum lines ran around the engine were for emissions control and weren't necessary. Do you know of any diagrams or pictures that show these different screws and throttle closer device?
 

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There are no vacuum lines connected to my carbs, that's how they were when I got them and it ran just fine without them. I thought that most, if not all, of the vacuum lines ran around the engine were for emissions control and weren't necessary. Do you know of any diagrams or pictures that show these different screws and throttle closer device?
107610

I found this image online of the balance tube, this is probably what you were talking about. Which would be the throttle closer adjustment?
 

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Why does it look on the left carb like, as the air starts to flow, that liquid (showing dark under the piston) seems to be evaporating and then reappears as the engine stops and piston comes down?
 

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Why does it look on the left carb like, as the air starts to flow, that liquid (showing dark under the piston) seems to be evaporating and then reappears as the engine stops and piston comes down?
I assume that's fuel being dumped by something weird in the carb. It could also be the dashpot damper fluid; I have ATF in it right now but I assume there's still some 20w left in there from before I drained it a few days ago.
The left carb is old and was functioning fine, I just sent the right carb off and got it rebuilt for $300. I didn't have these issues before I took the carb off so I'm peeved that its happening now, AFTER paying $300.
 

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There are no vacuum lines connected to my carbs, that's how they were when I got them and it ran just fine without them. I thought that most, if not all, of the vacuum lines ran around the engine were for emissions control and weren't necessary. Do you know of any diagrams or pictures that show these different screws and throttle closer device?
Vacuum lines aren't the problem here. It would be the gaskets that connect your carbs to the intake manifold or something like that. I've seen this before - just go smoke the engine to see if you have an air leak. Worst case scenario you don't have any vacuum leaks and it's something else, but at least you'll be able to tell us that and we can look for solutions elsewhere.
 

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107656
Dylan, here is a graphic from the 72 air Cleaner housing. The 2 ‘A’ screws are the base idle screws. B, C, and D are all supposed to be spring loaded screws for fast idle setting, emission control and high speed balance. Obviously your C & D screws have been replaced. For now just make sure they are backed out fully and not contacting. Then make sure both of your A screws are backed out fully and not contacting anything. Now each carb should be operating independently. It appears to me that ONE of your carbs, probably the newer one has the throttle waaayyy open and it takes the other carb along for the ride. That D screw on yours looks like it’s turned in way to far. I think once you get them separated and both idle screws turned down you can then get the idle set and then balance with the D screw at about 2500 rpm.
Also that juice when the piston drops is probably just excess damper oil. Regardless that you use you only need to have it between the two marks on the damper piston shaft. It’s not hurting anything if it’s too full. It will just suck it out until it gets to the right level and burn it off in the intake.
 

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Discussion Starter #14
View attachment 107656 Dylan, here is a graphic from the 72 air Cleaner housing. The 2 ‘A’ screws are the base idle screws. B, C, and D are all supposed to be spring loaded screws for fast idle setting, emission control and high speed balance. Obviously your C & D screws have been replaced. For now just make sure they are backed out fully and not contacting. Then make sure both of your A screws are backed out fully and not contacting anything. Now each carb should be operating independently. It appears to me that ONE of your carbs, probably the newer one has the throttle waaayyy open and it takes the other carb along for the ride. That D screw on yours looks like it’s turned in way to far. I think once you get them separated and both idle screws turned down you can then get the idle set and then balance with the D screw at about 2500 rpm.
Also that juice when the piston drops is probably just excess damper oil. Regardless that you use you only need to have it between the two marks on the damper piston shaft. It’s not hurting anything if it’s too full. It will just suck it out until it gets to the right level and burn it off in the intake.
I got it. It was such a stupid mistake that it took me forever to find it. It turns out we got the throttle linkage put on very wrong. You can see in the pictures where each individual throttle carb throttle is below the catch where it's supposed to be.
107681
107682


The car is running almost perfectly fine now; I've backed out both of the A screws almost all the way and the individual throttle adjust screws C and D are all the way backed out, but the car is still idling fairly high, probably around 1800 rpm (I can't tell because the tac doesn't work lol. One problem at a time)
I would think it would idle a LOT lower, but I'm not sure whats going on with it now. Thank you for your help, I'm not sure why it took so long to get that fixed.
 
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