rwd:

A method of making the Control Pressure (CP) adjustable (in the downward/rich direction) is to bridge the outgoing line (top of FD) to the return line (adjacent to main pressure regulator in base of FD). By including a needle valve in the bridge line the CP may be reduced, and the mixture made richer.
Consider installing a wide-band O2 sensor, and ideally, a recorder, so that you can see what the mixture ratio is.