M-1942 MOD – Minimal Effort to Blue Flame

M-1942 MOD Stove Outdoors

A 1944, M-1942 MOD Stove

 

Fettle – When used as a verb, “I am going to fettle this stove”, means to set in order, get ready.

So, you’ve got yourself an old stove and you’re thinking, “Let’s see if it works!” after all, if you don’t fire it up how will you know if there’s something wrong with it, right? Well, that would be one dangerous way to discover that the 70 year-old fuel cap gasket or valve stem graphite packing doesn’t seal so well and after you light it you discover, to your surprise, that   vaporized or liquid gas is escaping . . .  somewhere, and has just ignited, and now the whole stove appears to be engulfed in flames and you go scrambling for a shovel and some dirt or a fire extinguisher. You’re taking an unnecessary chance if the stove hasn’t been used recently, and there are a few easy things you can do to give yourself a decent chance at a good experience. Spend some time to fettle your stove before you fuel it and put a match to the burner.

SKLColorado disclaimer: You must take charge of your own safety and I will not be responsible for mishaps with your stove. I’ve rebuilt a lot of stoves and sometimes things don’t go as planned for me, so I cannot stress safety and precaution enough. What I describe in this blog post are things that I do, based on what I’ve learned over the years, however, I learn new things all the time and cannot possibly anticipate every situation or the condition of your particular stove. If you’re uncertain about anything, STOP . . .  contact me or one of the two popular forums, Coleman Collector’s Forum or Classic Camp Stoves and ask questions. Restoring and/or just using old Gas Pressure Appliances (GPAs) such as stoves, lanterns, lamps and irons are a hobby for some people and generally speaking they like to help other interested hobbyists.

First, let’s understand a little bit about the operation of the M-1942 MOD stove.

Side note: If your stove doesn’t have a legible instruction label or M-1942 MOD decal, consider getting one. Reproductions are available at either Old Coleman Parts or many times on ebay.

M1942_stove label_old and new

M-1942 MOD stove labels, reproduction (left)

The M-1942 MOD stove, unlike the M-1942 “wheel” stove, 1940s 520 or 530 stoves, is not an instant lighting stove. The instruction label affixed to the side of a stove says, “TO LIGHT – Clean tip by moving cleaning lever down and back up. Pump 20-30 stokes. Open valve slightly to fill priming cup ¾ full. Close valve. When fuel is almost consumed, open valve gradually to obtain a maximum blue flame.” The reason it doesn’t say pump stove, light match, open valve is because the M-1942 MOD is a stove that requires pre-heating. Instant lighting stoves of this era, and newer, have an F/A tube, that is, a Fuel/Air mix tube, which consists of an inner tube, an outer tube and a steel wire which goes down the middle of the inner tube and is operated by the main valve. When the valve is opened slightly, the outer tube, which has two holes in it, passes air from the air reservoir inside at the top of the tank, down to the fuel where it mixes and brings a fuel/air mix through the valve stem opening to the generator.

Side note: this is why you should never over-fill stoves with F/A tubes, which basically means most every U.S. made white gas pressure stove and lantern made from 1940 through today. Overfilling prevents fuel-air mixing, as fuel is forced down the mix tube instead of air.

FA_tube_when Cold

A typical F/A tube when cold

Once the vaporizer (a.k.a. generator) is warm, the user opens the valve, where the needle running down the middle is retracted allowing more and more fuel flow as the valve is opened further, until it is fully opened.

FA_tube_when Hot

A typical F/A tube when hot (fuel control rod is retracted)

So, why mention a design that doesn’t apply to this stove? Because the M-1942 MOD is different than other stoves and lanterns you may have used and, as such, needs to be operated differently. It has a single, straight tube which delivers a powerful flow of fuel right out the top of the vaporizer when the stove is cold and the tip cleaner is retracted. Attempting to light a stream of raw fuel will be an issue . . . a problem . . . possibly dangerous . . . . messy . . .  you get the point, don’t do it. Please follow the instructions.

M1942_fuel_tube

M-1942 MOD straight fuel tube (no control rod)

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M-1942 fuel tube (left), typical instant lighting F/A tubes (middle and right)

 

On the Coleman Collectors Forum, there are also some better actual Coleman drawings.

 

What is a vaporizer . . . what does it do? The vaporizer is a brass tube which sits atop the valve assembly, but below the burner bowl, and has a brass or stainless steel mesh/screen inside. It is where raw fuel turns to vapor and the screen helps heat the fuel and allows some space for the expanding gas. Here’s how it works. Following the instructions, you fill the priming cup ¾ full with raw fuel, close the valve and light. The vaporizer gets heated by the burning fuel in the priming cup. After thirty seconds, or so, the vaporizer is hot enough to turn the raw fuel still sitting in the vaporizer into vapor where the expanding gas exits the vaporizer tip and ignites. It’ll sound like the stove is already running at which time you can open up the valve to allow a continuous flow of fuel into the vaporizer for normal operation. Using gasoline for fuel, for which some people use the acronym RUG for Regular Unleaded Gasoline, would have been typical in the military, however, it seems to clog the screens badly. I would use RUG in an emergency, but until then I use white gas.

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M-1942 MOD vaporizer (left), installed in stove (right)

 

To pressurize the fuel tank, there is a pump. The pump happens to occupy the space where fuel is added, so the pump assembly is also the fuel cap. This is different than all other GPAs (Gas Pressure Appliances) that I’ve used where the burner is integrated with the tank (in contrast to stoves with separate tanks, like MSR Whisperlite or Coleman Denali, Fyrestorm, etc.), except the M-1950 stove. Unlike other pump and check valve designs, you don’t twist the grip (pump handle) before pumping and there is no hole for you to place your thumb over. Just begin pumping.

M1942_Pump_wText

M-1942 MOD pump, gasket locations

So, that’s how fuel gets from the tank to the blue flame.

One thing to look at right now, is the valve assembly clocking. Think of the valve stem with the black knob as the hand of a clock. Looking from the top of the stove, compare its position to that of the one shown in the photo. The one in the photo is the correct clocking, plus or minus a little, based on the stoves I’ve looked at and worked on. If yours is off by a lot, say 1/6th of a revolution or more, then that’s a sign that someone has removed the valve assembly. It doesn’t necessarily mean anything bad, it’s just something to keep in mind if things don’t go right. My first M-1942 MOD stove had a messed-up clocking and when I tried to tighten the vaporizer nut, I discovered the whole valve assembly moved. If I would have just fired up the stove, assuming I could even create any pressure, it’s likely fuel vapor would have seeped out of the valve assembly fitting that mates to the tank and caused unexpected flames.

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M-1942 MOD stove valve assembly clocking

 

Next, two things you should not do when fettling the stove for the first time (at least, not yet):

#1 – Do not tighten the tip-cleaner packing nut.

#2 – Do not tighten the valve stem packing nut.

Why not? Isn’t it a good idea to make sure nothing is loose or leaking? Yes, but . . . graphite packings, which consist of more than just pure graphite, can get brittle with age and from using the stove, or fire, etc., so this is a case of, “if it ain’t broke, don’t fix it.” Any further attempts to compress the packing could crack it and create a fuel leak where none existed before.

Next, four things you should do when fettling the stove for the first time, especially if you don’t know the last time the gaskets were changed and don’t know if the stove has been tested. You should:

#1 – buy a replacement fuel cap/pump gasket.

The fuel cap/pump gasket is under the lip of the large pump tube (see photo above). After removing the pump assembly, hold the pump tube in one hand and unscrew the cap until you can separate the inner pump tube from the outer pump tube. Use a dental pick, T-pin, or straight pin to remove the old gasket. I typically try to push the point of the tool into the old gasket at an angle and then try to lift it out. Sometimes this works quickly, sometimes it’s more difficult. Be careful to not damage any of the surfaces or the lip of the pump tube. Slide the new gasket down and press it into place. Make sure it sits flat.

#2 – buy and install the air-check gasket which is also called the NRV gasket, dot and pip (I don’t know why some call it a pip . . . ).

The air-check gasket resides in a small brass cup inside a fitting on the end of the larger of the two pump tubes (see photo above). Be careful when removing the fitting that you don’t lose the very small brass spring which goes with it (see pump post if your spring is missing). Use a 1/4” wrench to remove the fitting. If it’s too tight to remove it when holding the tube in your hand, use a pair of pliers, but use a thick piece of rubber or some layers of leather between the pump tube and the pliers so as to not tear it up with the teeth of the pliers. Also, grip the tube towards the fitting end and not in the middle where it has no support. Use a dental pick, T-pin, or straight pin to remove the old one. Install the new gasket in the cup and reinstall onto the pump tube. Re-tighten with wrench, but don’t over-do it.

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Use a piece of leather (this is from an old slipper), or piece of rubber from a bike inner tube to prevent damage

 

#3 – If you haven’t oiled the leather pump cup yet, now is a good time to do it. I use neatsfoot oil and work it into the leather after it soaks in for a few minutes.

Reassemble the pump pieces.

BTW – There are several places that sell the two gaskets, pump cup leather and even the valve stem graphite packing. One source who regularly stocks the parts is Old Coleman Parts and periodically, there are sellers on ebay.

#4 – Rinse out the tank with denatured alcohol or white gas.

To rinse the tank, don’t fill it up, but instead, just add a few ounces of either denatured alcohol (you can typically buy denatured alcohol at Home Depot, Lowes, etc.) or white gas. The tank is made of stainless steel, so it probably won’t have rust in there. The goal is to remove old dried fuel, dirt, gravel, etc. With the pump assembly installed, shake it around and then drain it into a fuel-tolerant container. Take a look inside the tank. Is there still a lot of loose sediment or dirt in there? Try again. If there appears to be a thick film of something on the bottom of the tank, I recommend adding several ounces of fuel and letting it sit overnight. Sometimes, it’s surprising what comes out of there and you sure don’t want some blackened old fuel crud to break loose and clog your fuel tube or something else. Spend some time to get it as clean as is reasonable.

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This dirt came out of an M-1942 MOD fuel tank

 

Those four items may not seem like much, but these days I don’t even bother adding fuel to the tank if I haven’t done them. I don’t think it’s worth catching a stove on fire or worse, when one or two dollars in gaskets and a little time would have prevented a mishap. If you’ve given proper attention to those four items above, you’re ready for the next steps.

#5 – Remove the burner bowl, using a ½” wrench on the nut which mates it to the vaporizer.

#6 – Add fuel. Four or five ounces ought to do for now.

#7 – Check that the pump creates pressure in the tank by cycling the pump 25 times. Then, open the pump/cap to release the pressure. You should hear the air escaping with a hissing sound. If you do, pump again to 25 and proceed to the next step. If you don’t, either the pump isn’t generating pressure or you’ve got a leak. Look for fresh fuel on any parts of the stove. If everything seems normal, ensure the main valve is fully closed, check the pump/cap gasket to ensure it is seated flat, check the air-check gasket and finally, check that the leather pump cup is not dry. After reinstalling everything and ensuring the pump/fuel cap is tight, try pumping again. Can you feel the resistance increasing as you pump? If so, check for pressure by opening the cap again. Once you generate pressure you can move on to the next step.

#8 – Be sure the tip cleaner lever is lifted up. When you pull the lever up it retracts the needle from the vaporizer tip. Pressing the lever down forces the wire at the end of the cleaning needle through the hole in the vaporizer tip. This is how you ensure the hole in the vaporizer tip is cleaned and free (mostly) of carbon. If possible, take a really close look at the vaporizer tip while moving the lever up and down. Can you see the wire moving? If so, great. If not, don’t panic, yet, but it is something to keep in mind for later.

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M-1942 MOD tip cleaning lever. The lever is down which means the needle is up and closing off the free flow of fuel

 

#9 – Be sure the tip cleaner lever is still in its lifted up position, then hold a rubber stopper or your finger firmly over vaporizer tip. Open the main valve and check for leaks around packings and vaporizer priming cup nut. Close valve after 20 seconds. If everything remains dry, you’re in good shape. If not, you need to fix the problem. If fuel is leaking around the tip cleaner packing, tighten it up using a 5/8” wrench. If fuel is leaking around the valve stem, tighten it by using a 9/16” wrench. If the fuel is leaking around the vaporizer/priming cup net, use the multi-tool wrench which came with the stove or a custom made ¾” wrench to tighten. If these things don’t fix the leak, do not continue on to the following section. You need to fix this. I plan to have a post on M-1942 Troubleshooting.

Homemade vaporizer wrench

A cheap 3/4″ wrench and a file is all I needed to make my own wrench for use at my bench

 

#10 – If you’ve gotten this far, it’s time to check the fuel flow. If you haven’t already done so, remove the stopper, and while holding a paper towel a couple of inches above the pot-support/windscreen, and protecting your eyes, you will observe the fuel flowing out of the vaporizer when you open the valve for one second and then close it again. If the stream shoots pretty much straight up for at least a few inches it’s good. If it doesn’t and just drips out, or doesn’t flow at all, you’ve got a problem. Recheck for pressure in the tank, and if you have pressure, it may be that the vaporizer is clogged with carbon or old fuel crud. If you don’t have any fuel flow, you can’t really go any further, so if you have pressure in the tank and the tip cleaner lever is lifted up and nothing happens when you open the valve it’s time to remove the vaporizer. Check out my post on vaporizers or my M-1942 Troubleshooting post for some guidance.

#11 – Replace burner bowl and align openings to pot-support arms.

If that’s all fine, you’re in pretty good shape, but we’re not assured of a properly operating stove, just yet. However, now, it is time to fire it up. First, some warnings about what to look for.

After I’ve worked on a stove, I take it outside to light it up on either a large, clear area of level concrete or in some dirt and preferably on a calm day. In either case I keep a shovel full of dirt nearby to toss on the stove just in case something doesn’t go right (fortunately, I’ve never needed it).

Wherever there is a seal, graphite, rubber, or metal-to-metal junction, you should check for fuel leaks after the stove is running (Many times I do this in the evening when it is dark because it is easier to see the flames). These would be the places we already checked when the stove is cold, but things can change when the stove gets hot. The basic rule is, if you see flames anywhere other than in the burner bowl, shut the valve off immediately if possible. If the flames are out of control, throw the dirt on the stove or use a fire extinguisher to put out the flames. The only exception to the flame rule, of course, is in the priming cup when preheating.

Well, here you go. Are you ready? Extend the pot-support arms upwards and make sure the steel ring drops down to prevent them from folding in again. If you forget to do this, when you open the valve to allow fuel to drain into the priming cup, you’ll discover fuel running down the arms instead, making another mess. Follow the instructions precisely and everything should go well. Tighten fittings as necessary. If additional tightening of the packings makes the problem worse, you probably need new packings (check out my M-1942 troubleshooting post). If it doesn’t go well, you might want to seek help . . . and no, I don’t mean help of the psychiatric kind for wanting to use an old stove!

Be sure to check out my previous posts on the M-1942 Mountain Stove or other stoves on this blog.

Thanks for visiting my blog.

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One comment on “M-1942 MOD – Minimal Effort to Blue Flame

  1. […] For a description of fettling this stove, see my blog entry, Minimal Effort to Blue Flame […]

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