likelihood of an occurrence of flashback improves co emissions and reduced the possibility of yellow tipping another embodiment of the gas burner system 2 is shown in fig2 it is to be understood that those elements in fig2 which correspond with similar elements in fig1 a and 1b will be numbered the same as in fig1 a and 1b in particular system 2 has a conventional gas mixing tube 4 with curved ends 16 a conventional cap 6 in this embodiment the autoregulating means comprises a diaphragm 14 diaphragm 14 is suitably supported in burner head 6 disposed substantially transverse to and spaced from the open end of mixing tube 4 an annular gap b is formed between the opposing surfaces of diaphragm 14 and annular collar 16 formed by the flared end of mixing tube 4 the effective cross sectional area of this gap varies as diaphragm 14 flexes in its cold state represented by the solid lines 14a in fig2 gap b is at its minimum providing a reduced effective cross sectional area for the gap consequently less primary air is entrained thereby reducing the likelihood of the occurrence of lifting as the burner approaches its steady state temperature of approximately 450 u00b0 f diaphragm 14 flexes to the position shown in dotted lines 14b in fig2 in this state effective cross section at.