Abstract: A fuel injection control mechanism for use in a fuel system is adapted to be connected between a fuel injection pump and a fuel injector. The mechanism includes a main passage, a chamber, a primary valve connecting the main passage to the chamber, a piston disposed within the chamber, a spring biasing the piston toward the primary valve and an adjustment compartment. A wall separates the chamber from the adjustment compartment, and a removable cap encloses the adjustment compartment. Apparatus is provided for adjusting the biasing force of the spring and travel limits of the piston. Adjustment can be accomplished to vary fluid characteristics in the fuel delivered to the injector without the need to disassemble the mechanism or disconnect the mechanism from the fuel system.