I have an earlier Petromax 821 in great condition. When I light it, it starts off brilliantly white on a Peerless mantle. It does get hotter over a short period and then starts to go dimmer with overburn on the mantle. The burner and Preston loop gets red-hot (like on all my Petromax’s). The J-tube gets red as well. Is this because of too big or too small an air gap between the J-tube and the jet?
You have a leaking jet, common problem with petromaxes. Tighten it up. Air gap on an 821 should be 7-9mm. As the lantern heats up, the loose threads are allowed to expand and kerosene vapor starts to leak and enriches the mixture.
Thank you! Let me give it a try. On a Optimus 930 with similar problems I replaced the J-tube with a new one and the problem was gone. These things are hard to explain!
It can also be caused by an overfilled tank, so the kerosene/air mixture becomes too rich. I've seen it on a petromax I had for repair.
@Tom Pedersen That is really strange, the fuel level of the tank should in no way effect the burn of the lantern, either it has fuel and pressure or none at all. Did you mean too much pressure? It may have an impact but doesn't seem to be an issue for me as I regularly overdrive my petromax.
@Dashwood Thank you for your answer. I have a user manual that says to only fill the tank 3/4 full, otherwise the mixture will be too rich. The problem with the Petromax I had for repair was solved by pouring a bit of kerosene out. The J-tube became red hot before I removed any of the fuel. After that the lantern ran perfectly. Maybe just luck. Tom
@Tom Pedersen I usually fill right to the neck because that's as full it can get without it spilling out.
The only relation with amount of fuel versus light output/under burn is: air pressure. If you fill the tank the way it is designed for, with the blue plastic funnel you can't overfill the tank. In reality this is just under the filling opening. Let say this is 3/4 of the volume, then you are left with 1/4 of air space under pressure. This 1/4 will rapidly drop in pressure when using the rapid and normal use of the lamp. Do you fill 1/2, you have 1/2 of volume of pressurised air. Off coarse temperature is also a factor like air pressure but let keep it simple ;-) And how accurate is the pressure indicator?
Hi all. I have tightened the jet, it had a small amount of play to fully tight. I also adjusted the air gap between J-tube and jet to 14mm, as per my other Petromax’s. This one has a little butterfly valve in the J-tube that was not completely “open”. I adjusted to open position. Lantern now running great! Looks like it was a combination of loose jet and air gap mostly. Not sure what role the butterfly valve has
@Pdebruyn The 821 should have an air gap of 7-9mm as it's a 250CP lantern. You will risk back burn as too lean a mixture will overheat the inner case. The butterfly screw is the mixture screw, use it to adjust the brightness, normally having it set wide open is ideal but in some cases, adjustments maybe required
Simple: to adjust the airflow to the correct setting. Maybe you should have mention and done this earlier. Often the paddle is burned away or removed. Using the screw: let the lantern light for a 30min., so it is hot, then adjust the screw looking at the reflected light, against a white background (wall piece of paper). Turning the screw would affect the air flow and therefore the light output.
@WimVe The optimal position of the butterfly screw seems highly dependent on the mantle used and it's geometry after burning in. Sock like mantles with a fine weave like to burn their brightest when it's fully open and some bulbous coarse woven mantles like the butterfly screw choked to burn their brightest.
Correct, this mantle shape will also influence the heat transfer to the coil. But its is who comes first: do you let the mantle shape itself with the valve full open or not? So the valve position is not dependant, the mantle shape is.
@WimVe I run the butterfly choked to ensure the mantle doesn't rip or overstretch when it inflates for the first time, then I adjust as necessary.
Oddly enough, I do the exact opposite when I'm burning in a new Tilley mantle - give it a blast at full pressure when I'm absolutely certain the vapouriser and burner are hot enough. That way, I'm sure the mantle will inflate properly to a nice round full shape first time. It's not a method I'd recommend to beginners though!
Ripping a new mantle would not be caused by the flame, simply because the pressure should be the same (more or less) all the time. Why would that not be? Unless you use not the correct pressure, jet size, mantle size. The mantle will shape itself according to the flame/pressure coming from the burner head. I have no idea how the flame should "overstrech" a mantle. Well yes when it is to small or the quality is not there (no shrinkage or to much of it).
David, Exactly what I found; shape the mantle while you can! That is with the first burn, with high pressure. I sometimes help the mantle burn, using a butane torch, so that all of the mantle turned to ash, including the fabric under the string on top, before I open the valve with a quick turn.
