Ok, I've been fettling with my Petromax 829/500HK, chinese model and I'm almost out of ideas. It does not burn right or bright enough. There is some flaming outside the mantle but not much. Tested different mantles, Helox, luxor.. same thing. Changed many necessary parts: all seals, brand new generator, needle, jet, burner head.. except the J-tube. Turning the valve open (pressurezed tank) without top hood, it shoots staright jet of petroleum up to 2 meters or so. I also cleaned the J tube, sealed the ends of fine tuning screw which is at vertical position. There is no leaks. Used exhaust paste for jet and mixing chamber/burner head. One thing I noticed: it burns clearly brighter without the top hood. I also had to adjust the air gap up to 18mm so at first it seems like running rich but why? This lantern would propably be fine using 250HK jet and mantle. So the last thing im concerned is the actual J tube. It seems to be fine, but is it really? Well there is tiny dent caused by the tightening screw but could it be causing such a severe burning problems? Also I have seen J tubes with and without restriction somewhere beetween 2-3cm from the fuel jet end. What is the actual purpose of this restricted part of J tube? Chinese J tubes has only very sharp edge around there, inside the tube. I removed that small edge and I thing it went even worse..