1Mix all Port Wine Marinade ingredients. Place pork in shallow glass or plastic dish. Using meat injector, fill injector container to 1-ounce line. Inject marinade into pork every 1 to 2 inches, pushing plunger down slowly. Refill container and continue to inject marinade until marinade is used. Cover and refrigerate 1 hour.
2Heat coals or gas grill for direct heat. Cover and grill pork 4 to 5 inches from medium heat 20 to 25 minutes, brushing with jam during last minute of grilling, until pork has slight blush of pink in center and meat thermometer inserted in center reads 160°F. Add plums to grill for last 4 to 6 minutes, brushing with jam during last minute of grilling, until hot
3Cut pork into slices; serve with plums.