Fire Maple FMS-118 Inverted Canister Sputter

[Ryan Bressler]

My wonderful wife recently got me a Fire Maple FMS-118 for my birthday. (She knows I like to play with odd gear). For those not familiar with this stove it adds a preheat tube and stainless legs to the recently reviewed fms-117T:

http://www.backpackinglight.com/cgi-bin/backpackinglight/olicamp_xcelerator_review.html

I've been doing a lot of testing (daily coffee and tea making) of the stove in inverted canister mode along with a windpro II and Jetboil sol (burner bought as spare part) with warm and chilled (1-10F) canisters in ambiant <20F conditions simulating harsh winter field use. No matter how much I try to ensure the stove is preheated I get a nasty sputter running the stove in inverted canister mode. I did a full write up of my testing including some short inline videos of the sputter on the hillmap blog:

http://blog.hillmap.com/2013/01/fire-maple-fms-118-sputters-in-cold-vs.html

Does any one here have any suggestions for other techniques or mods I could try? It is a cool little stove and I would love it if I could get it to perform as well as the windpro and sub in some ti or MYOG legs to have an ultralight winter setup. Anyone try brazing (?) in a longer preheat tube to a stove?

Thanks,

Ryan

[David Adair]

Just guessing, but it may be coming from an abrupt temperature change at the junction of the flex pipe to the brass pipe. You might be able to correct it by installing a thin piece of music wire inside the length of the feed pipe. This should make the liquid boiling off to gas more gradual and less twitchy. -Just a thought.

I was thinking about getting one of these stoves myself, so glad to see your post and looking forward to seeing if there is a good solution.

[Ryan Bressler]

Next time I have time i'll see if I can disassemble the thing enough to get some wire in there. Anyone know if the feed tube is just screwed in there and if it will require some sort of silicon tape to reseal?

I also wonder if the fact that both the cold and preheated fuel pass through the same block is just a flawed design that cools the preheated fuel back down (the WindPro separates them). There are also a couple of near right angles in the tube down there as well which could lead to some sort of weird cycle of pooling and bubbling liquid.

Finally, just visible in the second photo is a threaded hole in the base…I'm not quite sure what that is for.

[Roger Caffin]

The sputtering usually indicates that the stove is not yet warm enough. Bits of liquid fuel are reaching the jet. Yes, this is possible despite a 'long' warm-up if you go straight to full power – especially if starting in sub-freezing conditions.

Try running the stove at medium to low power for a few minutes. You can't lose this way, as medium power is still pretty hot and the fuel efficiency is higher anyhow. In fact, running at 'full power' is often problematic with any stove in serious cold weather until everything is quite warm.

Cheers

[Ryan Bressler]

Thanks for the info on preheating Roger. I did try to ensure what I considered excessive preheating before inverting (on the order of two minutes) and then turned the valve close its lowest setting before inverting…all of these things did help and you can see the difference in the two videos but the sputter is still very pressent.

Do you worry about preheating with the canister upright for this long burning off all of the propane while their is still liquid isobutane left in the canister?

One thing I would like to use this stove for is quick stops to melt snow on mid winter day ski tours and for this fuel efficiency is less of a concern then getting the job done before I freeze or run out of daylight…the msr requires no babying whatsoever and may be a better choice for my situation.

I'll make another attempt with even more preheating (it is now above freezing here but will get cold again soon) so if you have any other suggestions let me know.

[Gaute Lote]

What Roger said. Due to short length of the preheat tube on the FMS-118 thats sits in in the flame it requires a longer time to properly preheat:
http://i1239.photobucket.com/albums/ff517/gzcityshop/FMS-118/FMS-118-3.jpg

Compared with other burners with preheat tubes where the preheat tubes are in contact with the flame longer and therefore the fuel gets heated and converted to gas form in shorter time.

Primus Spider express – the preheat tube almost surrounds the burner head:
http://3.bp.blogspot.com/-ix49ggPBtc4/TdQmkvPCGuI/AAAAAAAADDU/BfKiWZtdAkg/s1600/P5145879%2Bcopy.jpg

Msr Windpro – preheat tube around 50-60 % of burner head circumference

[Stuart R]

As has been suggested already, the sputtering is caused by liquid fuel coming out of the jet.
The cause is not the temperature of the pre-heat tube (it only needs to be warm for the fuel to vapourise) or the length which is in the flame (it does not even need to be IN the flame to get warm). Indeed, if the pre-hea tube gets too hot you will see o different effect which gives an orange flame.
The cause is the internal volume of the pre-heat tube being too great. As the liquid fuel comes into contact with the metal it boils, so rapidly expanding bubbles are formed. The bubbles can expand so rapidly that they can push small amounts of liquid right through the tube before it can all vapourise.
The solution, if the stove can be dismantled, is to insert wire into the preheat tube to minimise the volume and maximise the internal surface area. Braided stainless cable as used for bicycle brakes (or gears) is ideal.

[Ryan Bressler]

Stuart R's interpretation and idea for a mod sounds promising and consistent with my observations. I checked and I'm heating the thing enough to get the block at the bottom too hot to touch.

I tried to disassemble the stove enough to get access to the preheat tube and it is quite easy to remove the feed tube from the block at the base of the burner with a small crescent wrench. It is attached with a threaded fitting and some white thread seal tape. The problem is that the preheat tube makes a small radius 90 degree bend immediately after exiting the block and i've been unable to feed any wire past that point into the tube. I tried a variety of stiff guitar strings and will see If I can dig up some bike cable but I expect that something quite bendable but stiff enough to feed will be required…I may have some copper wire around to try.

The preheat tube is also quite easy to bend so it might be possible to bend it into a wide radius, insert the wire and rebend it though i would worry about rupturing the tube doing this too much.

It is forecast get back down into the single digits (F) here by friday so I hoep to have some sort of mod completed to try by then.

Thanks again for all the ideas,
Ryan

[Roger Caffin]

> The cause is the internal volume of the pre-heat tube being too great. As the liquid
> fuel comes into contact with the metal it boils, so rapidly expanding bubbles are
> formed. The bubbles can expand so rapidly that they can push small amounts of liquid
> right through the tube before it can all vapourise.
Yes, the volume of the tube is a significant source of the problem, but there is a bit more to it. When the preheat tube is hot and the fuel is going through at a great rate, you can find that there is a tiny bit of liquid fuel at the core of the tube, insulated from the hot preheat tube by a thin film of gas.
The solution is to fill the core of the tube somehow. Yes, flexible cable is a good solution. Heavy copper wire is also a good solution. If you can't get the wire past the big bend at the top of the tube, try running with it only part way through. More of a problem if you can't get it past the 1st bend tho'.

Cheers

[Nick Gatel]

A few years ago I rebuilt a tent trailer and added some LPG (propane) appliances. As part of the rebuild I replaced all the copper LPG lines and added a couple. I was told that for natural gas and propane the "Yellow" tape or "Yellow" liquid sealant was required to meet building codes in the US for RVs and homes. When I went to the hardware store, they had both in "Yellow." White is for water lines.

Anyway, anytime I take apart fittings that use sealant, I always clean and re-seal. So if you take this apart you might want to check and see you use the correct material.

I am not an expert, just thought I would bring it up.

[Ryan Bressler]

I finally managed to find a wire I could fit into the preheat tube on the fms-118 thanks to the suggestions on this thread. A single strand from a stranded copper wire inserted to just past the upper bend (harder then it sounds due to the sharp bends…long nosed pliers and patience required). This seems to have fixed the sputter with a canister fresh from the freezer, we haven't had any really cold temps at the house to test it in.

Detailed how to write up and more pics:
http://blog.hillmap.com/2013/02/fixing-fire-maple-fms-118-inverted.html

[Hikin’ Jim]

Thank you for posting this. When is a pre-heat loop not a pre-heat loop? When it's poorly designed. I think the Chinese stove designers have got a ways to go yet.

HJ
Adventures In Stoving

[J C]

Just tried this on my FMS-118, can confirm that it solves the sputter somewhat (although the flames still flicker a bit the sputtering noise is gone). Bike gear cable didnt work for me because of the bend in the individual wires. Some flexible braided wire work better for me although I'm not sure it went all the way around the top bend. Might try again with copper sometime.

[Roger Caffin]

> not sure it went all the way around the top bend
Does not need to. Getting anything > 1 mm copper wire up near the bend and sticking out into the hose 10 – 20 mm should be great.

Cheers

[J C]

Did you mean <1mm? The tube isn't much bigger than 1mm.

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