My Optimus 930 350cp model is in overall great condition. I have taken it apart and cleaned. Vaporiser has been de-carbonised with a mixture of heat/quenching and citric acid treatment. Problem is the light output decreases slowly as it heats up. It initially starts out very bright and then slowly decreases in light output as it heats. This does not change despite pressure level. The J-tube eventually glows red and typical for this model, it runs very hot. The space between the jet and the airtube is 13.4mm currently. Could this be wrong?
Your lantern is having the 'back-burn' symptoms. It can happen due to a variety of reasons. The lantern might start-up fine but eventually gets dimmer even though you increase the pressure and operate the cleaning needle. The J-tube would quickly turn red-hot and might even melt through if left burning for longer. There is a chance that it would not happen if you were to light the lantern without the outer hood attached. (you can experiment it but this isn't a solution). There had been several threads in the past about them. Example, PX 828 300HK fire ball Depending on the exact cause, there can be several means to address them. Sometimes, although the exact logic isn't that clear, such problems had in the past, been solved by several members here with just the replacement of the j-tube(mixing chamber) alone. The space between the jet and j-tube is usually not the cause of such problems.
I agree with @MYN opinion. I experienced the same phenomenon with my 930. The problem was solved by replacing the J tube. Another lantern has had issues using a third-party brass nozzle. This is because too much heat is transferred to the J-tube. I hope your lantern improves.
There is no reason to swap the J-tube. Unless it has dents or is not original. Back burn has 99% of the times to do with less pressure. Less pressure comes from no pressure in the tank or blockage of the vaporiser and/or jet.
It does not over flame at all (makes no flames outside of mantle). The mantle just gradually glows less bright. I have cleaned the fount and the fuel is clear. The pricker needle works and the jet is clear. Can you confirm the air gap (between jet and airtube) on your lantern?
I have heard about this several times and have also had to deal with it myself twice, where the J-tubes of Optimus lanterns start to glow. Unfortunately, in all these cases, the only thing that helped was to replace the mixing tubes. Adjusting the distances, replacing the vaporizers, mantles, burners, and fuel did not help. It even went so far that the problem "migrated" when the inner hood was replaced, and could finally be narrowed down to the mixing tube. It almost seems as if the combustion is already taking place in the J-tube due to some kind of catalytic reaction after a pretty well-defined while. This is also not noticeable by a change in the burning noise, as is often the case with flashbacks from the burner. The mantle simply becomes darker and darker after a while and the J-tube starts to glow.
@Martin K. , the dimensions of the J-tube should be identical so shape wise there is no reason. For me "needling' is the way to solve a back burm into the J-tube. Besides getting the pressure right.
As I mentioned, the only possible reason in my opinion could be a kind of catalytic reaction in the J-tube. However, I don't know how this could be related to the material. I even tried to lacquer the inside of one tube with high-temperature silver paint, but this also had no effect. A swap of the J-tube and nothing else, and the problem disappeared immediately.
Well it is an interesting idea to think about. New parts are smooth, old parts can be rough, hence more air resistance. But also when you chance the parts, the geometry changes, you never get parts back in the same place.
@Pdebruyn You can start with a 15mm air gap between the jet top surface and the inlet of the J-tube. Only adjust it if you need to optimize it further later. Anyway, it is not the source of the problem in this case. On many occasions, we have, at some point of time, experience this phenomenon..(red hot glowing j-tube accompanied by dimming of the light outputs.). If there are any physically obvious defects along the mixing chamber/j-tube or broken screens or burner caps, then replacing or repairing those might help. Eliminate all causes that might lead to a reduced fuel-air velocity in the mixing chamber. Examples, low fount pressure as WimVe mentioned or any possible leakages(no matter how small) in the generator/vaporizer and the jet. A skewed or unsteady fuel stream from the jet can sometimes(not so common) initiate the problem. In such cases, either clean the jet or replace it if the orifice seems enlarged or defective. As I had already mentioned earlier, this problem can be baffling, and can be caused by seemingly unexplainable reasons, whereby a mere replacement of the old but seemingly defect-free J-tube might well solve it. As Martin K had already experienced, I myself is no exception. Had a few hypotheses about it as Martin did. On such occasions, there had been no obvious physical defects on the 'problematic' J-tube. I had also theorized the catalytic phenomenon similarly to Martin. The old J-tubes might have undergone some metallurgical changes over the course of usage. Perhaps loss of zinc or similar. So much so, that it might induce autoignition of the fuel-air mixture within the tube. It might not even be exactly defined as a 'backburn' with which, the combustion point eventually retracts back into the tube from the mantle/burner. Instead, a 2nd ignition was triggered within the tube whenever the J-tube and fuel-air mixture reaches some undefined higher temperatures, shortly after start-up. I had managed to curb the ignition cause due to overheating of the tube or fuel-air. This was by means of wrapping the J-tube with an asbestos cord as insulation. It worked and the lantern could operate as normal. In addition, and without such modifications, we can also sometimes stop the glowing phenomenon by just running the lantern without the top outer hood attached. It will then not be overheated due to better venting of the heat. However, it was baffling to me why would all these be required at all??. It was never required when the lantern was still new as originally designed.
I'm learning a lot from everyone's thoughts. I've had a few repairs done on my 930, but it only happened with a certain J-tube, so replacing the J-tube works best.
Tried running it without the ventilator on, no issues! J-tube does not get red hot. I suspect that these lanterns are made to run very hot, especially the 350cp model, and that the ambient temperature may have been too hot here to run this model when I had the issue. Ambient temperature was a balmy humid 34 degrees Celsius with little wind. It will become a "winter warrior"! Could this be a contributor?
While you can experiment without the hood and temporarily get around the problem, that would still not be a real solution . I only suggested that for anyone who might be interested to examine further, the underlying cause leading to such issues. Anyhow, I would doubt the ambient temperature is significantly contributive to the phenomenon. I live in the tropics too and the ambient temperature is similar to what you have in your location. A new, and suitable J-tube would likely solve your problem. (The temperature under the hood would still be just as hot as before, but you won't get a red glowing J-tube).
I measured the gap between jet and airtube intake at 13.5mm. I changed this to 11.5mm. The J-tube got red hot even quicker. Next attempt is to make there gap as big as the thread allows, hoping for about 14.5 - 15mm. All this while I wait for a new J-tube. No airlocks observed and pumped to what I felt was maximum.
While waiting for a new J-tube, you might just wrap the existing one with an in insulative asbestos cord if you have any. I've experimented with that and it did work for me. It'd prevent the J-tube from attaining the threshold temperature which triggers the internal burn. At least, as a temporary solution with whatever available at hand. Asbestos might not be available in many countries at present. It has been phased-out due to health concerns. It is still easily available from many regular hardware stores in my location. Any ceramic, heat-resistant insulation material would do if asbestos isn't an option for you.
So I took the advice of replacing the J-tube on this problematic 350cp Optimus 930. As you can see from the pictures, the replacement supplied by The Base Camp differs substantially from the original the lantern came with. It cannot adjust for gap between gas jet and airtube intake. It also does not take the adjusting flat screws of the original. The airtube portion that goes over the gas jet is also 1mm wider than the original, which meant I had to make the gap wider with a Dremel. The material seems to be quite a bit thicker than the original. The gap between gas jet and airtube is now fixed at 16.5mm. The end results appears to be a solved 930! I did not change the jet or pricker needle so the advice of the J-tube was correct. Thank you very much for there advice!
Congratulations on the successful light up. As to why a seemingly physically non-defective J-tube could be at fault, remains opened to be examined and explained.
