Jim,

If your duty cycle reading is fluctuating, then it appears the car is in closed loop and functioning normally. When checking it, you need to take an average reading which is easier with a multimeter like a Fluke that has that function. As you are probably aware, right around 50% is what the factory states as desirable. But much like yours, mine tends to hunt around but usually is within a 5% tolerance with averaging. Just watching the readings can result in fluctuations from 30% to 70%.

Black soot does indicate a rich mixture so you will need to lean it out. Mine use to stain the garage floor until I got it correctly adjusted.

The 3mm allen wrench the correct tool with only slight movement of the adjustment resulting in significant changes.

Tinker