How the Coleman FA tube functions

Discussion in 'Fettling Forum' started by Schnuppiepup, Feb 21, 2021 at 10:45 AM.

  1. Schnuppiepup

    Schnuppiepup United States Subscriber

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    I thought that the FA (fuel/air) pickup tube from an old Coleman suitcase stove of mine might be malfunctioning. (A 425C two-burner, perhaps? I don't know. I now have only the tank.)

    So I searched for information on exactly how these tubes work, and found what I wanted on the Coleman Collectors Forum website.

    Below we see a page of text and a diagram from what appears to be a piece of Coleman literature from about 1960.

    FA tube 1.jpg

    FA tube 2.jpg

    https://drive.google.com/file/d/0B4_5mgrc2IliZTNOc3BORWRPVzQ/view

    Fascinating! I had always thought that in "instant-light" mode, with the central rod down and blocking the orifice at the bottom, only fuel vapor from the fount went up and out the "generator". Instead, atomized droplets of fuel, plus air and vapor, go up and out. Apparently, the small amount of liquid fuel initially present in the pickup tube goes up the inner tube and is not replenished, except, maybe, by a tiny amount that sneaks through the orifice alongside the rod; any that does sneak by gets atomized by the air/vapor rushing up through the inner tube.

    Also, I had always wondered how the air/vapor flow gets blocked in liquid "run" mode so that only liquid fuel then goes up the tube. Well, when the central rod lifts and opens the orifice at the bottom, fuel rises up into the pickup tube to match the level of fuel in the fount outside the pickup tube. Equilibrations of pressure keep the level at that height. No additional air/vapor then can pass into the air inlet hole at the top of the outer tube. Magic.

    The three time-sequence diagrams below show these stages clearly (Coleman Fuel Valve Operation Diagram - The Coleman Collectors Forum).

    FA tube 3.jpg

    Ignore, in the diagrams, the spring and the way in which the valve stem controls the up-down movement of the rod (and ignore the parts laid out in the lower right). This graphic shows the FA tube in a Coleman lantern. In a typical old Coleman stove, one moves the rod up or down with a lever. In all other respects, the FA tube in a stove is essentially identical. Note how, in the middle diagram, in "instant-light" mode, with the rod down and blocking, mainly air/vapor passes down between the outer and inner tubes and then up the inner tube. Then in "run" (liquid) mode (third diagram), the fuel levels in the fount and between the tubes are equalized, thereby blocking the passage of air.

    So, what did this knowledge tell me about my perceived "problem" with my FA tube? I was concerned that the outer tube was "loose": it was able to twirl around within its crimped connection at the top. This crimped connection is shown about 1/3 of the way down in the second image I posted. I believe now that this "looseness" doesn't matter. No fettling required.
     
    Last edited: Feb 21, 2021 at 10:56 AM
  2. presscall

    presscall United Kingdom SotM Winner SotY Winner Subscriber

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    Useful compilation of information @Schnuppiepup. An early lesson in this hobby is to understand how the device works but pulling the equivalent from a Coleman 226 backpacking lantern had me stumped recently.

    Superficially it looks similar, equipped with concentric tubes. Fuel inlet in the lower tip, air inlet hole in the outer tube just below the threads.

    8F3EC84C-60E2-4A4E-9D82-A38B1060B657.jpeg


    There’s no fuel/air metering rod however and the fuel inlet orifice is much smaller than in the usual component.

    C78A2401-A2C3-4F99-A0FC-BDAD021F571B.jpeg


    Several minutes (ok, maybe an hour or several) later I believe I get it.

    Here’s the lantern components.

    72DA7CAE-4ADE-42D0-AD6D-E5D7C841C641.jpeg


    The answer’s in the actions of the eccentric block. When that’s in its highest raised position (control knob pointing to ‘Off’) the O-ring shuts off fuel flow to the generator ...

    B2DE17E5-0089-4663-87CE-95E629740274.jpeg


    ... by mating up against the chamfer where the valve riser bore decreases in diameter.

    854A118D-FABB-4139-ADC3-1E8A9AB364AA.jpeg


    With the control knob turned to ‘High’ the lower end of the eccentric block enters the space in the threaded portion of the fuel/air tube, not enough to shut off fuel flow but sufficient to dampen flow and prevent surging.

    ADF8DC1B-AD50-4D23-8E73-C20B2E171847.jpeg


    In use the fine, almost pinhole, fuel inlet restricts (meters) the fuel, which mixes with air admitted by the outer tube’s air inlet. The equalisation of pressure you spoke of in your analysis limits the air reaching the fuel (so not de-pressurising the air space too rapidly) which becomes a combustible fuel/air mixture when vapourised in the generator.

    It’s not as good a system as the device equipped with a fuel/air metering rod. I can make a direct comparison with another Coleman miniature lantern, a 222, which has a f/a rod in the pickup assembly. The 222 starts easily, the 226 sputters and is slow to fire up (the pinhole fuel inlet makes the mixture very lean on fuel). Subsequently, the 226 requires re-pressurising with the pump more frequently.

    Nice work @Schnuppiepup!

    John
     
  3. Schnuppiepup

    Schnuppiepup United States Subscriber

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    Thanks for the kind words. Now see what you've done, though? So graciously, with your clear description of yet another fuel pickup assembly? You've motivated me to fettle on a Coleman lantern of mine that I've simply used and used and used. I used it something like 60 hours in 2020. (I have six lanterns ... I think. I lose count. It may be an illness.) Now that I've been prompted to think about it, this lantern was performing oddly: it surges perceptibly. But enough about me and lanterns ...

    ... Anyway, what your posts show, here and at Coleman 226 fuelling, is that Coleman created more than one design ― at least three and perhaps more designs ― for mixing fuel and air/vapor for priming in their various appliances over the years. Their lantern plumbing differs from their stove plumbing, and the Coleman Apex stove plumbing differs, of course, from that of Coleman suitcase stoves. (My collection/hoard includes a Coleman Apex II stove (!) that has sat on a shelf in new, unfired condition for 16 years and that I have no desire to use and fettle on). The exploration of "design space" in pickup tubes by the Coleman engineers is a fascinating topic, not to mention the theory underlying each pickup tube itself and whatever fuel+air mixing it does. You make a foray into how performance varies among pickup tubes: another fascinating topic of analysis.

    Indeed.
     
    Last edited: Feb 22, 2021 at 8:34 AM
  4. presscall

    presscall United Kingdom SotM Winner SotY Winner Subscriber

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    Yes. That variation in performance was underlined recently in a post by Doc Mark where we’d been talking about the utility of a ‘backpacking’ lantern. No better to note a variation than Doc, since he has lengthy experience in using Coleman marque products (and MSR, and many, many others!) so has used early production 222 lanterns right through to late ones, which by then (model 222A) will have had the same type of valve and pickup assembly installed in my 226, with no fuel/air metering rod and a ‘pinhole’ fuel inlet.
     
    Last edited: Feb 22, 2021 at 9:36 AM