Abstract: In a liquid fuel burner of the spill return type, comprising an atomizer 4 for discharging liquid fuel and a rotary change-over valve 25 for conditioning the burner into a "circulate" mode (FIG. 1) in which no fuel is discharged from the atomizer 4 but circulates through the burner housing 2 and a "fire" mode (FIG. 2) in which liquid fuel issues from the atomizer 4, the rotary change-over valve 25 is adjustable into a plurality of different positions providing different rates of discharge of fuel from the atomizer 4 when the burner is conditioned into its "fire" mode.