A device for the electronically controlled lean out of mechanical fuel injected engines comprising a wide band air fuel ratio sensor and a printed circuit board (PCB) connected to the wide band air fuel ratio gauge/controller. The printed circuit board (PCB) is connected to the wide band air fuel ratio sensor's power, ground, and signal wires. The computer controlled stepper motor is connected to the printed circuit board (PCB). A variable valve spool is retained in a fuel block and connected to the computer controlled stepper motor. Rotating the variable valve spool continuously adjusts and controls the air fuel ratio of the engine in real time by regulating the amount of fuel returned the fuel tank and the amount of fuel delivered to the barrel valve in a mechanically fuel injected engine. A jet can be used in combination with the fuel bock to further fine tune the fuel flow.