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2100 Carb: Auto Vs Manual

Discussion in 'Intermediate CJ-5/6/7/8' started by Chilly, Apr 16, 2019.

  1. Apr 16, 2019
    Chilly

    Chilly Active Member

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    Mar 12, 2007
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    I have two 2100 carbs. One is 73 for auto, other is 77 for manual. Obvious difference is the deeper cover for the 2-stage power valve on the 77. What woulf be the difference between a carb intended for auto vs manual? Just the initial tune? Maybe slightly faster idle for the auto?

    I'm surprised my 73 SuperJeep has a 3DA2 tag. Its the right build date for a Super Jeep era. Figured a coincidence if replaced later. Or maybe Im misunderstanding what the "M" and "A" stand for...
     
  2. Apr 16, 2019
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor

    Medford Mass USA
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    Yes, M and A are what you say. My book only covers '74 and newer carbs, but between the putatively comparable 4DM2 and a 4RA2 there is lack of a dashpot on the A carb and different power valves. Main jets are part of the carb body and not available separately, so those could be different.
     
  3. Apr 18, 2019
    Chilly

    Chilly Active Member

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    Ok, turns out I had the answer if Id looked more closely at the manual.

    Manual idle passage is 0.057" vs 0.0465"
    Manual is jetted one size richer (47F vs 46F)
    Manual PV at 8.5" Hg vs 7.5" Hg
    Manual spark port higher at 0.025" vs 0.042" above closed throttle plate.
    Manual set with choke open .130" vs .110"
    Auto choke set 1 notch richer than auto.
    Dashpot on manual, not on auto.

    And here I though carbs is carbs. I dont think it much matters. Only a couple of things are baked into the carb machining. I can adjust my idle screws to compensate for different idle passage restriction, and spark port location doesnt matter because I run vac adv on manifold vacuum. Everything else is setup and parts (jets, PV).
     
  4. Apr 21, 2019
    Chilly

    Chilly Active Member

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    This gets stranger all the time. Not only is my "auto" carb of the correct build season for my manual Jeep, but now I am also certain it had a dashpot on it when I got it. I took it off because it was in poor condition but Ive decided to put one back on. Its also jetted like manual carb.

    Just for fun I'll poke at some of the built-in differences to see what model it was machined as. Maybe the tag got mixed up sometime in its life.
     
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