Tip: If using a charcoal grill, open the bottom vent completely. Light a charcoal chimney starter filled with briquettes. When the briquettes are covered with gray ash, pour them onto the bottom grill grate and push them to one side. Adjust vents as needed to maintain an internal temperature of 350-400 degrees F. Place the top grate on the grill. Place the foil-wrapped potatoes on the top grate over the side without coals. Grill, covered, until tender, about 1 hour, 30 minutes, flipping and rotating the potatoes halfway through grilling.