Abstract: An adjustment mechanism for allowing fine-tuning the amount of fuel to a Diesel engine by an operator of the engine is provided. A module conveniently within reach of the operator of the engine allows the operator to tune the engine independent of any throttle setting while the engine is operating. In one embodiment for an electronically controlled Diesel engine, an electrical input representative of barometric pressure to an electronic engine controller is modified to be under control of the operator, who can adjust the input independent of the throttle to vary injected quantities of fuel according to the instant conditions under which the engine is operating.