I acquired a 1924 Primus No2 about two years ago. I finally got around to starting to fettle it this week. I found that the tank held its pressure but that the burner was not letting even air through. It was a bit of a struggle getting the roarer burner off and I had to apply some heat to the area just under the spirit cup. I then removed the nipple. I soaked the burner in a warm citric acid solution for half an hour, took it out, scrubbed it with a nail brush, rinsed it well and then back to the citric acid for another half hour and then rinse. No luck - the burner was still completely blocked with carbon. Out came the blowtorch to try the 'heat-and-quench' method. I usually grasp the hexagonal burner nut with mole-grips and then hold the mole-grips with a thick oven glove. After about 10 to 12 times of heating and quenching in a bowl of cold water, lots of carbon had been loosened and expelled from the burner. I then heated up the burner to dry out the insides and let the burner cool to room temperature. Next comes the 'tap-and-knock'. This involves tapping and knocking the burner to loosen the remaining carbon. In this case I tapped and knocked onto a Kleenex tissue so you could see the results. As you will have noticed, lots more carbon came out of the burner. After about 10 minutes of this, I then went back to the 'heat-and-quench' method for about another 5 quenches, and then back to the 'tap-and-knock' method. Both released more carbon, but eventually no more carbon appeared. By the way, the burner is now completely clear and can be blown through easily. The reason I have given this account here is because I do not think that stove novices realise how much carbon can accumulate within the burner head. Do you get yellow flames on your 00, No1 or No5, etc? Carbon may be causing it.