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Thread: Quick Carb Tuning Question

  1. #1
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    Default Quick Carb Tuning Question

    After searching, every carb tuning guide tells you how there should be no vacuum leaks in the system (1971' 240z). it also states to do with this the air cleaner assembly disconnected.

    Now I may be missing some very large step here… But with the aircleaner assembly off, aren't all your vacuum hoses disconnected (Meaning HUGE leak), minus the brake booster hose?

    Clearly i'm misunderstanding something.

    The reason I ask. I finally got my carb cleaned out, and now idles at a high(With air cleaner assembly off), but smooth 1100-1200 RPM. I then start researching how to reduce the idle, and came across people with brake booster issues causing a bad idle etc. I believe I have booster issues as well, and I am just trying to correlate all my issues before I go and start tinkering more.
    Last edited by HuD 91gt; 04-14-2014 at 06:55 PM.

  2. #2
    Brian
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    I think you can plug the brake booster line off for your tuning purposes and then hook it back up before you drive it again. If it idles well but when you step on brake it drops down, your booster or hose going to it is most likely leaking.

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    The hoses into the back of the air filter aren't vacuum lines. They all vent fumes from the float bowls and the engine to be burned. Vacuum lines are usually found coming from the base of a carb or the intake manifold.

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    Ok, so blocking the booster line will simulate a working brake booster. I can understand that. I assume all the other hose attached to the air cleaner assembly (Float bowl vent, Air injection system, crankcase vents) don't matter?

    Thanks for the reply!


    EDIT: Thanks Mark. So the only real vacuum system is the brake booster. All makes much more sense. What would I do without this forum
    Last edited by HuD 91gt; 04-14-2014 at 07:13 PM.

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    There is one more vac. line from the base of the front carb to the vac. diaphram on the dizzy.

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    Registered User Stanley's Avatar
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    There's a bunch of other vacuum activated stuff on the balance tube, all emissions-related. The "flying saucer" thing, the anti-backfire thing, and (I think) the "hat" thing, all use vacuum. If you remove all that stuff, the holes need to be plugged (BSPT thread). Those holes are handy if you want to hook up a vacuum gauge.

  7. #7
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    Basically all you need to do is make sure any air that SHOULD be going through the carburetor isn't going AROUND the carburetor.
    Any lines hooked up to the air cleaner are already upstream from the carbs and therefore irrelevant as far as tuneups are concerned. However, anything hooked up to the balance tube is downstream of the carbs so if there are cracks in those lines or other faults letting unmetered air get into the engine it'll lean you out and give you an erroneous tune-up.

    The brake booster doesn't need to be unhooked for tuneups if it's working properly (eg not leaking a bunch of air into the engine)
    1973 240Z w/ roundtop carbs, 280zx e12-80 distributor, 280zx alternator, late model Altima junkyard electric fans. 115 Blue Metallic & white side stripes.

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    For some reason I thought the brake booster was vented to the valve cover, which really made me wonder why this leaking would do anything to the carb. It all makes much more sense now. So, I was right. I was clearly misunderstanding something. ha

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    My '73 has brake booster running to the balance tube, PCV running to the balance tube, and a valve cover breather line (not a vacuum device, per se) running to the air cleaner box (not a vacuum source as far as carbs are concerned, all upstream)
    Click image for larger version. 

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    1973 240Z w/ roundtop carbs, 280zx e12-80 distributor, 280zx alternator, late model Altima junkyard electric fans. 115 Blue Metallic & white side stripes.

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    Where did you get your brake booster vacuum hose? Is it a cheaper alternative to the reproduction model at the theZstore?

  11. #11
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    On the car when I bought it.
    1973 240Z w/ roundtop carbs, 280zx e12-80 distributor, 280zx alternator, late model Altima junkyard electric fans. 115 Blue Metallic & white side stripes.

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    Registered User Stanley's Avatar
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    So I'm not the only one who used a semi-automatic drill to modify an air cleaner.

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    Also like that when I got it
    1973 240Z w/ roundtop carbs, 280zx e12-80 distributor, 280zx alternator, late model Altima junkyard electric fans. 115 Blue Metallic & white side stripes.

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