Primus 25 mystery

Discussion in 'Fettling Forum' started by mackburner, Nov 17, 2019.

  1. mackburner

    mackburner United Kingdom Subscriber

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    This has me puzzled. I have just acquired this Primus 25 in fair condition dated AD and appears to be all correct BUT it doesn't work. I get a Bunsen flame OK but it will only burn inside the riser tube and cap so the cap gets red hot but there are no flames from the multi holes. Looking at the parts for 25 it has the correct 4140 burner with the hex nipple, 4678 inner cap and 4206 outer cap. However I discovered that fitting the smaller 4205 outer cap will work and very well. I simply do not understand why it will not burn right with the correct cap. As I am not a stove expert I suspect I am lacking some vital information here. Help anyone! ::Neil::

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  2. Radler Switzerland

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    @mackburner
    I think your inner cap is too small for the large outer cap. If the space between the inner and outer cap is too wide, the speed of the gases is too low. The gas speed in the jet, the intake of air into the mixing tube, the thermic expansion of the mixture in the burner chamber and the speed in the holes of the outer cap are depending from each other.
    I own a Primus No.25 from 1932 with a silent burner with standard size outer cap. Maybe some of the users of the larger 3 pint stoves just wanted a longer use without refilling fuel, not a higher heath output (i.e. for keeping large pots hot during a whole day).

    I wrote some words about this here: classiccampstoves.com/threads/silent-burner-outer-holes.39192/#post-411255

    Best regards
    Radler
     
    Last edited: Nov 17, 2019
  3. mackburner

    mackburner United Kingdom Subscriber

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    Ah yes but that inner cap seems to be unique to model 25. It is the only one I can find in the parts lists with the riser tube as part of the cap. That inner cap will not fit any other model as far as I can see. I have to assume it is therefore correct. From the parts lists again the outer cap 4206 is unique to model 25. So I appear to have the correct parts. I agree the inner gap is too large which is why the gas flow is too slow and it burns back. As you can see the stove works very well with the smaller outer cap but it shouldn't according to the parts listed for the model. This burner also has a unique nipple. In the parts it shows as a hex instead of the more normal two flats and this burner does have that hex nipple. So all the parts are right so it should work perfectly. I was wondering if the parts were modified at some time. The outer cap I have was a new one. The guy who sold me the stove had made sure it was complete and correct and that cap had never been burnt. So did Primus change things after this stove was made in 1940 ish.

    All of this is not vital as the stove will function and I am not dependant on it for any purpose but it puzzles me still. ::Neil.
     
  4. kerophile

    kerophile United Kingdom SotM Winner Subscriber

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  5. Simes

    Simes Subscriber

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    Just a passing thought regarding the nipple as you suggest this may also be unique to the 25.

    If it's original then quite possibly it could be quite worn with the consequential increase in gas flow and reduced speed in the cap gap leading to the under burn. The small outer cap allows the gasses to actually get through the holes before igniting.

    Does the 25 have a standard jet size of 0.32?

    It's something I would have looked at put it that way.
     
  6. kerophile

    kerophile United Kingdom SotM Winner Subscriber

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  7. presscall

    presscall United Kingdom SotM Winner Subscriber

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    0.4mm jet orifice on that burner
     
  8. Simes

    Simes Subscriber

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    Thanks John, I didn't do a search hence the ?.

    And from George's excellent recollection as always, it seems like an issue with this cap configuration. Or at least sensitive to any wear or misalignment if I've read it correctly.
     
  9. Lennart F Sweden

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    Looking at your pictures it's obvious the large outer cap has too large diameter to fit either the inner cap or the burner - the smaller cap looks like a fit but I think there was a discussion where it was stated the original cap for that burner is between in diameter.
     
  10. mackburner

    mackburner United Kingdom Subscriber

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    The smaller cap is not a fit. The bottom of it sits on the bottom rim of the inner cap but those outer caps should sit on the burner. If you look the outer has a notch which I assume means it should lock onto the burner but this burner does not have anywhere for it to clip in. Makes me think there was a change in the years after this stove was made.

    I see this same burner and parts were used on model 10. That seems to be a missing model in a lot of catalogues. Still it still gives the same part number for the inner and outer caps and the jet.

    I think the jet is OK. I am used to worn jets in pressure lamps and they tend to give a rich mixture and yellow flame but this burns blue so I assume the jet is good. ::Neil::
     
  11. Lennart F Sweden

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    Some outer caps were updated with a rim but retaining the old part number - this may be the case here.
    The notch could fit the lighting device found on early Optimus 182 where the pilot flame comes up right beside the outer cap.
     
  12. Mike the stove

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    It looks like your inner is too small Neil. Primus made three inner caps with the fitted inner tube, #4678, #4679 & #4682.
    The smaller #4679 inner cap is used with the 5 row holes cap #4209 which is the same outer diameter as the 4 row holes cap #4205 which is why your stove works reasonably with the #4205 cap.
    I think you need the larger inner cap #4678 so that the edge of the outer cap #4206 sits on the rim of the inner.
     
  13. mackburner

    mackburner United Kingdom Subscriber

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    Did they now? That explains much so thanks. I am not so familiar with stove bits but a smaller inner cap has to be my problem. Now I just need to find a larger inner cap. ::Neil::