Place the yams in a large pot with enough salted cold water to cover them. Put the pot over high heat and bring to a boil. Cook until the yams are tender enough all the way through that they pierce easily with a fork. Total time from cold to cooked should be about 45 minutes, but this will vary based on your pot and your burner. Drain the hot water and refill the pot with cold water to halt the cooking. Reserve the potatoes until time to grill.
Prepare the grill for direct grilling over a medium-hot fire.
Stir together the ketchup and Sriracha. Reserve.
Combine the sugar, cayenne, five spice, salt and pepper in a small bowl. Slice the yams lengthwise into 6 wedges each. Lay out on a baking sheet and toss them with olive oil. Sprinkle the spice blend all over and then toss again for an even coating.
Place the seasoned wedges in the direct grilling zone. Cook each side until nicely marked, slightly caramelized and the wedges are heated through, about 8 minutes total.
