How to Make It
In a spice grinder or clean coffee mill, grind the chipotles, garam masala and cinnamon to a powder. In a large nonreactive baking dish, combine the spice mixture with the grapefruit juice, honey, sugar, vinegar, salt and 1 tablespoon or water, stirring to dissolve the sugar and salt. Add the pork and turn to coat with the marinade. Refrigerate for 2 hours, turning occasionally.
Light a grill. Using a damp paper towel, coat the grill with the oil. Grill the pork over a medium-hot fire, turning, until cooked through, about 15 minutes. Transfer to a cutting board and let stand for 5 minutes.
In a small nonreactive saucepan, boil the marinade over moderately high heat until reduced to a flavorful glaze, about 4 minutes. Slice the meat 1/4 inch thick and serve with the grapefruit glaze.