Your duty cycle will be stuck that high if the thermal cutout switch (under the intake manifold in the center) is grounded, or the pressure switch is grounded (the blue switch on the firewall). Also 84s have a pressure differential switch (black switch toward drivers side on the firewall). If it was stuck on and grounded it would cause a high duty cycle.
You could check these by attaching your dwell meter and then unplugging the wires to each of these sensors. If the dwell drops that sensor is faulty. The wire for the thermal cutout switch is in the engine harness, so if you harness is disintigrating a short in the harness could be grounding out your sensor.
It sounds to me like one of these sensors is causing the lamda sond unit to stay in "open loop" mode.