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Jittery Vacuum Gauge Reading

Discussion in 'Early CJ-5 and CJ-6 Tech' started by wheelie, Jun 29, 2019.

  1. Jun 29, 2019
    wheelie

    wheelie beeg dummy Sponsor 2019 Sponsor

    York, PA
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    So, I plugged the vacuum gauge into manifold vacuum after installing the new Motorcraft 2100 (haven't driven it yet, just running it in the garage). I had pretty low vacuum before, with the Holley carb on it but could not identify a vacuum lack around the carb anywhere. Now, the vacuum gauge reads much better, though still a tad low. That might be a timing issue which I have not yet addressed. I used to think that the timing was way off with the low vacuum and a typically hot running engine. Now I think it's off a little but not horribly. Still have to check/adjust it.

    Anyway, the gauge is now very jittery, which it was not, before, with the Holley. A jittery vacuum gauge supposedly indicates worn valve guides. This odd fire 231 has less than 10k since rebuild.

    So, first, why would this symptom suddenly appear? Was the previous low vacuum hiding the jittery gauge?

    Second, it was my understanding that these engine do not have replaceable valve guides. Not sure I'm right about that but, that's what I remember. At under 10k, it seems odd that thy would get worn already.

    Finally, is it really even that much of issue to be concerned about? I'm not interested in pulling the heads off this thing and having them gone over again, with knurling or whatever the repair might be. It's not a daily driver and honestly, sees under 1000 miles a year, though I'd like that to increase but, it probably won't.

    Next move for me is check the timing and get it right but, I'm curious what you engine guys think about the situation as it stands today, with the jittery gauge and it's potential affects, long and short term.

    Thanks folks.
     
  2. Jun 29, 2019
    PeteL

    PeteL Member Sponsor 2019 Sponsor

    Hills of NH
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    Tried a different guage?
    Are you sure you have pure manifold vacuum, not a partial, timed, or ported outlet affected by carb action?
     
  3. Jun 30, 2019
    wheelie

    wheelie beeg dummy Sponsor 2019 Sponsor

    York, PA
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    I believe I do, Pete. There are two plugs in one of the runners of the manifold. I pulled one of those plugs and installed a brass fitting to attach my gauge to. Seen to the pass side rear of where the carb mounts in the picture below (before the new fitting was installed).



    With the Holley, I tee-d into the line that went from the PCV to the base of the carb to use the gauge. That option no longer exists as the 2100 has no port at the base for the PCV.

    I do have another gauge I can try but, I can't imagine what might have happened to this one since last summer, that would have messed it up.
     
  4. Jun 30, 2019
    CJ Joe

    CJ Joe Truckhaven Tough!

    Vista, CA
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    I'll profess that I'm not an expert here. I would think that since you are now reading vacuum from within one of the runners it is affected by the #6 intake valve. Your more accurate reading would be in the plenum just below the carb.
     
    timgr likes this.
  5. Jun 30, 2019
    wheelie

    wheelie beeg dummy Sponsor 2019 Sponsor

    York, PA
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    Ok. I'll buy that, Joe. Reading from one runner instead of a more steady, centralized source. Makes sense.

    Unfortunately, these are the only available ports, AFAIK. There is nothing else since the 2100 itself has no port for the PCV. It appears that the PCV would, in a (Ford) factory application, be routed to the air cleaner assembly.

    I'm open to suggestions if anyone has any.

    It had been suggested that I drill and tap the block off plate where the EGR once resided. Have to tap the correct side of that. The vacuum side. See post #27 of the thread below.

    Carb Bolt Pattern-225 Vs 231
     
    Last edited: Jun 30, 2019
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  6. Jun 30, 2019
    timgr

    timgr Jeepin' Nerd Sponsor 2019 Sponsor

    Medford Mass USA
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    The PCV must go from the crankcase (typically on the valve cover or a location in the intake manifold) to the manifold vacuum.

    The carburetor should either have a PCV port at the base of the carb, or an insulator between the carb and manifold that accepts the PCV hose. The connection to the air horn is for clean filtered air going through the choke stove and into the choke cover.

    This is what I would expect -

    [​IMG]

    There is an insulator that I used to find at the junkyard that has a port in it, but I don't see an equivalent in a quick net search. There may be a vacuum port to connect the EGR to, but it typically is ported vacuum, not manifold vacuum. The EGR does admit exhaust gas to the intake manifold, but I expect there won't be a tidy way to block the exhaust and use the vacuum connection... maybe.
     
  7. Jun 30, 2019
    Norcal69

    Norcal69 Resident Taco Maker 2019 Sponsor

    Cottonwood, Ca
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    When you crack the throttle barely does it firm up? My vacuum reading is fluttery until throttle is applied. I am hooked to manifold vacuum.
     

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