When flames have all died down and coals are covered in white ash, place pork on side of grill away from coals. Put lid on grill with vents open one quarter of the way.
Roast, adding a large handful of new charcoal about every 30 minutes, until pork is done, 1 to 1 1/2 hours for medium-well. To check for doneness, insert an instant-read thermometer into center of roast, close to but not touching bone: 150°F is medium; 155°F is medium-well; and 160°F is well-done.