The duty cycle meter is controlled by engine vacuum. The more you push the go button (the throttle) the less vacuum there is in the intake manifold. The less vacuum there is, the higher your duty cycle meter goes. Well, when you shut off the engine, the duty cycle is going to read 100% because when the pistons aren't pulling any vacuum the intake manifold sets out to atmospheric pressure (or no vacuum at all) and the duty cycle meter tops out. If it's all over the place with the car running, I would imagine that indicates a vacuum problem.