A couple of years ago, when I was still gathering my backpacking equipment, I bought this heavy and rather expensive liquid fuel backpacking stove – Optimus Nova. The big advantage over MSR that everyone uses that it has metal fuel pump, that hopefully won’t break in the most important moment, and that it should have bullet-proof Swedish quality.
It was working fine fore me so far. We used it in the Wrangell trip a lot. It is a bit finicky to use – you have to prime it properly, turn on the valve correctly, etc. But I guess as any tool one needs to know how to use it. I can’t say that I was very proficient with it, but I was learning. However, I have to admit that I rarely cleaned the needle.
Now, on the two recent winter trips, both of them, the stove just refused to work on the second day. It would burn a bit and then it will be out. Specifically it would happen on the second day in the morning. Maybe there is some problem with cold? I was quite disappointed. This was supposed to be a very good stove. Bullet-proof. Working in all conditions. Now this?
So I contacted Optimus for help. Viyasan kept saying that there should be two filters – one on the tip of the plastic tube inside the fuel bottle and the other at the end of the fuel hose and that I was missing the second one. Optimus people said that there is only one filter.
the placement of the filter in earlier models was at the end of the fuel hose, in the connection to the stove. Now the filter is placed in the intake hose on the pump. There was always only one filter but the placement was changed. The reasoning behind this was to filter the fuel before it entered any part of the stove system.
The fuel filter performs 2 functions:
- Filter impurities larger than 60 microns. This protects the spindle from premature build-up of tar which could cause the stove to seize up and become completely blocked at the jet nozzle.
- As a pressure regulator. During normal and optimal use the filter acts as a buffer between pressure in the bottle and pressure in the fuel line. This serves to make the handling of the flame more smooth, However, when the filter gets blocked, it has the exact opposite effect.
Since the filter is quite dense (meaning the pore size is quite small) it can get clogged up quite quickly. Even from one use to the next. Typical symptoms of this are:
- A sudden drop in performance
- Sputtering and unsteady flame
- Pumping overpressure only seems to make things worse
There is also very interesting information about the workings of the stove in cold. It came from Optimus himself. It seems, from it, that the operation of this stove is much more complicated than I thought.
OK, extreme cold. Well, these issues have more to do with the kind of fuel you use than with the filter. The Teflon filter is stable, well below freezing.
White gas used in camping is mostly n-Heptane. It has a flash point of -5°C (meaning it will not turn to vapor below this temperature) it has a vaporization temperature almost the same as water (apx.100°C) What this means for field use is: more energy is required to start, and more importantly continue, a reaction. This is where pre-heating the stove correctly becomes important.
If the burner jet area of the stove does not reach and maintain a temperature more than 100°C the stove will not function. If the temperature of the jet area is just barely enough, the stove can get caught in what we call a “flame pulse loop” Meaning it is quickly cooling and heating and the jet nozzle area is just barely hot enough to vaporize the fuel. As it heats the fuel the brass cools down, the small amount of vaporized fuel ignites, heats the burner cup, which sends heat back to the jet vaporizing another small amount of fuel, before cooling down again slightly. This pulsing can then take a long time to dissipate and bring the stove to full operating temperature. We call a “filter pulse loop” the pulsing effect caused by a blocked filter.
So to avoid this, make sure you pre-heat the stove well, especially in cold weather. The nova is made with a lot of mass in the area of the jet, this means it may be a little more tricky to start the stove in cold weather but once going, this mass stores enough energy to help the stove continue a reaction well below -30°C. Something few of our competitors can claim. Do you use the windshield? This will also help the stove get up to and maintain a proper operating temperature. Also, I know of some people who sleep with their fuel bottle in their sleeping bag to warm up the fuel and make it easier to light. This works especially well with Butane gas canisters.
So that should be the end of it. I disassembled the stove completely, cleaned up the parts, and washed the filter is the warm soapy water as Optimus suggested. I still need to clean the end of the nozzle, but this can be done later. And the stove should be good to go. At least I hope so. I’ve read a lot of reviews where people were having problems with it. It ceased working in some conditions, cold, high, etc. I guess I will have to take care of it for it to work well all the time. That fuel needle is really small. Perhaps 0.2mm in diameter.
The stove has to be cleaned once a season. Filter washed in warm soapy water. Best way to clean the stove is with a dry cloth. Keep any O-rings and leather lubricated and occasionally (once a season) clean the spindle tip with soap and a toothbrush to remove any tar. The needle has to be cleaned before each use.
Some good ideas about backpacking food.