Abstract: A tamper-proof dual spring controlled air fuel control for a compression ignition type internal combustion engine is provided wherein fuel is supplied to the engine cylinders in response to the pressure of the air in the intake manifold. A pair of oppositely biased springs controls the movement of a stem valve in response to intake manifold air pressure exerted on a flexible diaphragm located between the oppositely biased springs. The stem valve includes a plunger within a barrel in the air fuel control housing in the engine fuel pump. The cross-sectional profiles of the plunger and barrel are designed to meter precisely a controlled amount of fuel to the engine fuel supply system as the intake air pressure increases and to reduce this metered flow as the intake air pressure decreases. Two plunger and barrel assembly embodiments having different profiles are provided. A class fit between the barrel and plunger of the present air fuel control wherein the clearance must not exceed 0.000075 to 0.