MSR Wisperlite question

10:31 p.m. on September 21, 2008 (EDT)
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I went to the high country this weekend and some trouble with my whisper Lite.

We went to cook dinner and the stove would not build pressure and I had to fill and light the cup 3x before it would run. Then the flame was very "Lazy", for lack of a better word and took over 40minutes to boil water. The next morning, however, it fired right up and ran like a champ.

The only thing I can come up with is I may have overfilled the bottle a bit and it would not build complete pressure..

Other factors for dinner, but same for breakast were
10.5K elevation
below 40F but above freezing.

Any Ideas?

11:11 p.m. on September 21, 2008 (EDT)
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3,956 forum posts

Hi Westy,
Here is a link to the user manual, it is a PDF file.

Your assessment is probably correct, I have better initial performance with my bottle filled to 2/3 or slightly higher, but not full. The fuel bottles from MSR have a fill line on them, and require 20 - 30 strokes to pump up.

I have no experience with my stove over 8000' so maybe one of the other guys can advise on that. In temps below freezing I put a cozy on my bottle to shelter it from the wind, I'm not sure how well that works really, but it must make some difference I would think.

It's possible that the jet was clogged, and was cleared by you moving the stove around before the second use. These stoves have a "shaker jet" that is kept clean by shaking the stove, but usually my stove gets shook pretty good on the hike in and during set up.

Also possible that the pump cup was not making good contact with the pump tube, this happens sometimes after periods of storage due to the cup drying out.
If you felt normal resistance while pumping, this is probably not the problem, but if you operated the pump with little resistance, then the pump was not building pressure.
Sometimes this problem remedies itself with some use, and sometimes you have to disassemble the pump assembly and lubricate the cup with mineral oil. My stove came with some oil in a little white tube, I lube it once a year and usually have no problems.

I really haven't had many problems with my stove and I've had it 13 years now.
But maybe someone else here can tell you of something I'm not aware of.

I'm sure it is something simple.

1:32 p.m. on September 22, 2008 (EDT)
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6,005 forum posts

First thing is that the Whisperlite, like all pump-pressurized stoves, does NOT "build pressure" during use. The only pressure to push the fuel through the fuel line is what you pump in, and that decreases as the fuel is pushed out to the burner and the air space expands (as they teach in high school physics P1 x V1 = P2 x V2, so as the volume increases, the pressure drops). Some stoves, like the Svea/Optimus 123, are heat pressurized. That is, the priming and cooking flame transmits heat via the stem of the burner to the fuel tank, which continually vaporizes fuel to keep the pressure up (that's why you should be very careful about windshields for stoves with the burner on top of the tank - you can overpressurize them by the heat to the point the safety valve releases).

Your problem is definitely NOT related to altitude or temperature. I have used Whisperlites up to 17,000 ft and temperatures well below 0 (down to -30 that I have a measurement for).

It may be indirectly related to having the fuel bottle overly filled. When you prime (and I will note that most people overprime), you let a little of the pressure out, since the volume of tha air space increases. What you should do is after lighting the priming flame, and occasionally during cooking, you should add another 10 to 20 strokes to keep the pressure up. After the bottle is down by 1/3 or more, you won't notice the drop-off in pressure as much. According to discussions Jim S and I had with MSR a decade or so back, you can not overpressurize the MSR bottles using the plastic pumps.

But ... as trout says, your pump "leather" (it is actually a kind of synthetic rubber in certain versions of the MSR pumps) can become dry, or it can get distorted when it is very cold, so that the pump is not very effective. That is, you get no pressure buildup as you pump. Lubricating, as he said, will usually cure this. If you do not have the pump oil with you, you can take the pump stem out, smear some saliva on the "leather", and then pump away. I have doubts about this being the problem, though, because with just a little experience, you can feel the pressure building as you pump (it gets harder to push the pump in as you pump, as trout noted).

Also check the jet, as trout said. If your Whisperlite is the later "shaker jet" version, then shaking the burner before priming will clear the jet. If it is the earlier version (more than about 20 years old), you need to get one of the cleaning needles. Do not use any random wire to poke through the jet, since with time that will enlarge the hole, and you will get poor performance.

Oh, yeah, if you have the Whisperlite International, it came with 2 different jets. One (usually marked with a "G") is the one to use with "white gas", while the other (marked with a "K") is for kerosene. Using the wrong jet will give very poor performance.

How recently have you replaced the O-rings on the pump assembly? Worn or cracked O-rings will allow the pressure to bleed off. Get an overhaul kit and replace all the O-rings and maybe also the pump spring and ball.

It is also possible that you have a pinhole in the fuel line. But that should show up as fuel leakage.

10:13 p.m. on September 22, 2008 (EDT)
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HaHa, I thought Bill might have a little to add, I did not know that you could not over pressurize the bottle.
If you can use the stove down to -30 I guess I can leave my cozy at home.
On the subject of over priming, the first time I primed my stove it looked like the Olympic torch!

11:51 a.m. on September 23, 2008 (EDT)
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To clarify a bit -
With some stoves, if you pump too much, the pressure will blow the flame a little away from the burner ("silent burner" type, not the "roarer burner" type). The cure is to not open the valve as much.

With the old Sigg fuel bottles, the metal was thin enough that you could pump the pressure high enough to bulge the bottom of the bottle. The newer fuel bottles from MSR, Optimus, and Primus are stronger, so this is not a problem with the plastic MSR pumps. I don't know about the metal pumps that Primus and Optimus use, though I suspect that the limitation is the cup (the "leather", which is sometimes actual leather, and more often a synthetic rubber or plastic). One reason to follow the manufacturers' statements to "always use fuel bottles made by [the manufacturer of the stove]". Other reason is the slight difference between US and European threads.

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