The present disclosure is directed to a flow control device, such as a regulator, for controlling fluid flow between an inlet port and an outlet port, and having an internal adjustment mechanism for varying the outlet pressure of the device, including the pressure required to close the device. The device may include first and second components, such as a body and a bonnet, demountably attached to form a housing of the device and defining an internal device cavity. A piston is disposed and movable within the device cavity, and in fluid sealing relationship with the inner surface of the device cavity. The components and the piston are configured to define a channel within the device cavity placing the inlet port in fluid communication with the outlet port, and with the piston being movable between a closed position preventing fluid flow and an open position allowing fluid flow to the outlet port. The internal adjustment mechanism may be in the form of a wheel that may be rotated in either direction to increase or decrease the outlet pressure at which the piston will move to the closed position. A vent port offset from the longitudinal axis of the housing may provide access to the interior of the device for manipulation of the adjustment mechanism.