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Thread: Do I have a vacuum leak?

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    Default Do I have a vacuum leak?

    I have a 72 240z. I just bought it. I found the carbs would not always turn off the choke. I bought two used carbs on ebay, rebuilt them and put them on the car. I adjusted the jets to 2 and 1/2 turns out. The car would not start. I looked at the old carbs and found the rear carb was set to 6 turns out. So I set my rear card to 5 turns out and the front to three turns. Then the car started and ran well. I still have to balance the carbs and tune the carbs. Also my idle is around 1000 and it is set as low as it will go. Is this normal? Do I have a vacuum leak? How do I track this down?

  2. #2
    Registered User 70 Cam Guy's Avatar
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    Default

    What I did to chase down my vacuum leaks was by spraying carb cleaner around the carburetor mounting surfaces, vacuum lines, fittings, along the manifold gaskets, and very importantly, the throttle shaft on the carbs. The throttle shafts on my first set of carbs leaked and it meant I could not balance the carbs and the idle would not settle below, say, 1200 rpm. The bushings are common wear points and wear out of round (oval) creating air leaks. The idle would only come down with the throttle adjusting screw backed out fully on the front carb, using the rear carb to control idle. Basically this means they are grossly unbalanced at idle and the mixture is haywire because you've really shut down the front carb by closing the throttle screw.

    You will need to verify you have no vacuum leaks before attempting the synchronize/balance procedure. The rpms will bog if the carb clean finds a vacuum leak. Some people say it raises but mine bogged and I could have stalled the engine had I kept it on the leak. Either way, you will likely hear a change in engine rpm.

    Looking at the amount of turns you have on the mixture screws, that sounds like the float height could be off. You really need to make sure you don't have vacuum leaks before fiddling with the float height.

    Factory manual can be found here: http://xenons30.com/reference.html

    Also, the instructional DVD from http://www.ztherapy.com/ is supposed to be very good. Bruce Palmer, proprietor of ZTherapy, is a regular contributor to this board, and will likely have advice for you if he sees this thread.

    Aside from what I've said, read through the threads in this section, there is a wealth of information here. It's really how I become familiar with the carbs. The FSM helps to understand the carburetors as well.
    Last edited by 70 Cam Guy; 07-01-2009 at 12:21 AM.
    -Andy
    1973 HLS30-166105
    RB20DET swap, HKS GT2530 turbo, HKS Actuator, Hallman MBC, ebay intake manifold, ebay dump pipe, straight pipe, 14 psi boost

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    Default

    I tried the carb cleaner, but once the mist hit the hot exhaust, I had a white cloud of steaming carb cleaner. It did not ignite. Is there a way to do this safely? As far as the float goes, I tries 10mm and 14mm both acted the same. One place said a 71 is 10mm and all others are 14mm. The car is a 72 but the carbs are from a 71. Which is correct?

  4. #4
    JDM esprist's Avatar
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    Default Proper way to spot vacuum leak

    Quote Originally Posted by cswistock View Post
    I tried the carb cleaner, but once the mist hit the hot exhaust, I had a white cloud of steaming carb cleaner. It did not ignite. Is there a way to do this safely? As far as the float goes, I tries 10mm and 14mm both acted the same. One place said a 71 is 10mm and all others are 14mm. The car is a 72 but the carbs are from a 71. Which is correct?
    Carb cleaner can be used, but it could damage the paint in engine bay and makes burn-marks on hot engine components everywhere.
    Easy and better way is to use small propane gas from Home depot. *under $20) It it comes with torch nozzle, that is the one. Carb cleaner has to evaporate while spraying requid.

    Esprist

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    beandip beandip's Avatar
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    Default

    If you spray the carb cleaner with the small red tube it comes with , you can direct the stream exactly where you want. No point in shooting it on the manifold. As for your question about the year of the SU, for all practical purposes all the '70 through 72 are the '72 is a 3 screw but performance and use and adjustment is the same. If you had to turn the adjustment out more than 3 turns , you have either a major vacuum leak or are grossly out of balance. You are using the bake-o-lite insulators correct and with gaskets?
    Gary
    I'd rather die while I am living than live while I am dieing. CZC 1887 IZCC 12602 Member of NorthWest Z Car Club

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