Engine Advance related to Vac leak?

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  • Rusty76
    258 I6
    • Jul 15, 2018
    • 331

    Engine Advance related to Vac leak?

    I've been stumbling, stalling when I give my Jeep a little gas off idle.
    This has lead me to advance my timing from 8-10 to 15-19. I think my TSM is 8-10. At 15-19 engine seems to operate better. 15-19 Advanced seems high?
    Would a vacuum leak require the engine to need more advanced timing?
    I have not looked for a leak yet. Just more curious than anything..
    I don't think I'm getting any pinging and my cruising temp is 170 ish up to 190 when I park. 15-19 Advanced is initial timing at idle with the distributor vacuum line plugged.
    Thanks!
    1976 Jeep Wagoneer
    Rebuilt, 360, TH400, QT.
    Still not running. Soon!
  • MysticRob
    350 Buick
    • Nov 26, 2019
    • 808

    #2
    Invest in a good vacuum gauge and ensure you know what vacuum reads.

    General rule of thumb for any dizzy setup, advance is usually backed down from the factory for emissions purposes, not necessarily for performance. 3-5 degrees more advance than factory is pretty typical across different engines as a sweet spot for increased performance from what I've read over the decades.
    --Rob--
    1988 Jeep Grand Wagoneer / Baltic Blue & Tan

    My build thread:
    https://forums.ifsja.org/forum/tire-...er-restoration

    My Howell TBI Install How-To:
    https://forums.ifsja.org/forum/tire-...rb-e-o-d-452-2

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    • tgreese
      • May 29, 2003
      • 11682

      #3
      What carburetor do you have?

      The accelerator pump on the carburetor takes up the slack when you open the throttle. Opening the throttle causes a drop in vacuum which starves the engine for fuel until the vacuum catches up with the new throttle opening. The accelerator pump provides an extra shot of gas to overcome this momentary fuel starvation.

      Failure of the accelerator pump is common for these engines with the 2V carburetor.

      The TSM has excellent drawings and discussion of the carburetor circuits and how they work.

      Check for leaks too. A lean condition from a leak could make the pump shot inadequate to overcome the stumble. You can buy just the accelerator pump diaphragm, and it's easy to change without removing the carburetor. However, a complete carb rebuild might be in order if you've never done it.
      Last edited by tgreese; 10-28-2020, 09:52 AM.
      Tim Reese
      Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS, hubcaps.
      Browless and proud: '82 J20 360/T18/NP208/3.73, Destination ATs, 7600 GVWR
      Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
      GTI without the badges: '95 VW Golf Sport 2000cc 2D
      ECO Green: '15 FCA Jeep Cherokee KL Trailhawk

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      • Brynjminjones
        258 I6
        • Jun 11, 2017
        • 475

        #4
        I have similar symptoms on one of our other vehicles, which I think is related to a vacuum leak around the throttle shaft.

        It idles okay, drives fine if I punch the gas harder, but stumbles like mad if I gently toe-in to the throttle.
        I then noticed that my throttle shaft feels a bit loose and is externally wet with gas.
        1991 Grand Wagoneer - Hunter Green. All stock. Rebuilt 360, .030" over with Melling MTA-1 cam.

        1998 Cherokee (XJ) 4.0
        1997 Grand Cherokee (ZJ) 4.0
        1974 Ford F100 390

        Comment

        • tgreese
          • May 29, 2003
          • 11682

          #5
          Usually a loose throttle shaft will make the idle speed vary. It's not so much the leak around the shaft as the change of the throttle plate alignment with the throttle bore.

          I would look for other sources of leak, or as above, the accelerator pump. Seems unlikely that you could have a large enough leak in that area to cause a stumble. Maybe...
          Tim Reese
          Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS, hubcaps.
          Browless and proud: '82 J20 360/T18/NP208/3.73, Destination ATs, 7600 GVWR
          Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
          GTI without the badges: '95 VW Golf Sport 2000cc 2D
          ECO Green: '15 FCA Jeep Cherokee KL Trailhawk

          Comment

          • letank
            AMC 4 OH! 1
            • Jun 03, 2002
            • 4126

            #6
            Originally posted by Rusty76
            I've been stumbling, stalling when I give my Jeep a little gas off idle.
            This has lead me to advance my timing from 8-10 to 15-19. I think my TSM is 8-10. At 15-19 engine seems to operate better.
            15 - 16 is fine at sea level... increase with altitude, 1degree per 1000' or about

            What happen if you floor it... ? if it picks up power then time to test all vacuum related devices...

            1) vacuum advance at distributor, disconnect hose, and connect your handheld vacuum pump. Make sure that within a 2" of Hg, vacuum is holding up... increase to 5" or 7" it should hold it without major drop for over 1 min, a new one can do it for over 4 min. (That was my issue 2 weeks ago)

            2) slight brake booster leak

            3) vacuum controlled air cleaner flappers

            4) heater control system

            5) QT control, but you can hear the hissing if the knob is leaking

            for ease you can disconnect all the accessories from the vacuum ball and plug the outlet of the ball...

            a quick check is to spray brake cleaner while the engine is idling, a vacuum leak will aspirate the solvent and the idle will increase significantly...
            Michel
            74 wag, 349Kmiles on original ticker/trany, except for the rust. Will it make it to the next get together without a rebuilt? Status: needs a new body.
            85 Gwag, 229 Kmiles. $250 FSJ test lab since 02, that refuses to give up but still leaks.

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