Prepare the grill for cooking. Heat until the coals are uniformly ashy gray in color, about 20 minutes.  Scrub the sweet potatoes under running water with a vegetable brush. Cut them into 1/2 inch thick slices. Brush both sides with the melted margarine, sprinkle lightly with salt and pepper. Arrange the potatoes on the grill rack, turning and basting occasionally with margarine, until the flesh is soft when pricked with a fork, about 20 minutes. Makes 4 servings