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Thread: Vacuum System Question

  1. #1

    Vacuum System Question

    Greetings All,

    A couple weeks ago I had to replace a bad VAC pump on my Sport. After
    describing the symptoms (no movement of vac gauge needle, and no spin
    up of DG or AI), mechanic removed the pump and found the shaft was
    sheered and the pump seized. $460 for a new pump and installation, the
    vac gauge stands right up at '5' and instruments spin up nicely even at
    fast idle. (it was barely making the green at idle before, run up was

    Anyway, a few days later I wanted to learn were all the VAC hoses and
    components were located and followed the system through. I looked
    through my maintenance manual and could not find a diagram of how the
    system should be hooked up. Where would one find this? The one thing
    I did notice was the VAC gauge had only ONE hose connected to it. I
    did not notice any hoses laying around that might have fallen off. I
    would think that this open port would be a place were unfiltered air
    would enter the system. I also read (I think on BAC) that many of our
    planes have "pressure" vice "Vacuum" systems. I asked the mechanic
    about that and he said the system is definitely hooked up as a VACUUM
    system. (I have not been able to ask him about the gauge hose yet) I
    am hesitant to keep the system running if it is not put together right.

    Any thoughts appreciated and many thanks in advance

    '78 Sport 150
    Ser MB 901

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  2. #2

    I'm not completely sure but I think only one line goes to your vacuum gauge. I remember when we had my bird gutted for restoration and having to move the vacuum gauge up to the old clock hole because the new Super Clock was too long to fit. Anyway, I recall having a discussion as to which hole to hook the vacuum line. One hole is for a pressure system and one for a vacuum system. I don't think having the other one open matters since there is a diaphram between them which when deflected measures the "pressure" difference. As a side note, having the vacuum gauge up top came in handy during a vacuum pump failure at night (may not have noticed it as quickly as I did). Anyway, good luck!


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