A variable firing rate burner with a throttle valve assembly that regulates fuel and air intake while maintaining a substantially constant fuel/air ratio over a wide fuel/air flow rate range. The throttle valve assembly includes a pivotally mounted throttle plate that seats on an input manifold, the throttle plate being moved by a driver between low and high firing rate positions. Fuel and air are drawn through the throttle valve assembly and into a mixing chamber by a blower and directed to a flameholder where ignition occurs.