A system for maintaining constant pressure, such as at a mosquito fogging nozzle, by electronically controlling the speed of an engine that is coupled to a blower. A pressure sensor measures the pressure provided by the blower and provides a pressure signal to the electronic engine controller. The engine controller is of the proportional-integral (PI) type and utilizes pulse-width modulation (PWM) techniques to control an electromechanical actuator attached to the throttle of the engine to vary the engine speed. Constant pressure is maintained despite changes in the ambient temperature, humidity and barometric pressure. A safety circuit prevents engine over-speed if the pressure signal is lost. An over-speed circuit monitors and controls the engine speed at higher RPMs.