Adjust the acetylene knob until the flame no longer produces black smoke. Now, slowly open the oxygen knob. The flame will change color as more oxygen is added. The next section describes how to adjust the flame, but be aware: If the torch starts to make a humming or buzzing sound at any time, shut off the oxygen immediately. This sound is the result of the flame burning inside the torch tip, a dangerous condition usually caused by gas pressures set too low.