Place Soda Pop-and-Soy Marinade in a shallow dish or large zip-top plastic freezer bag. Pierce pork loin roast several times with a knife; add roast to marinade, and turn to coat. Cover or seal, and chill 4 to 6 hours, turning occasionally.
Light one side of grill, heating to 350° to 400° (medium-high) heat; leave other side unlit.
Remove roast from marinade, discarding marinade. Pat roast dry, and sprinkle with salt and pepper. Tie with kitchen string, securing at 2-inch intervals.
Place roast over lit side of grill, and grill, covered with grill lid, 5 minutes on each side or until browned. Transfer roast to unlit side, and grill, covered with grill lid, 1 hour or until a meat thermometer inserted into thickest portion registers 150°. Remove from grill, and let stand 10 minutes before slicing. Garnish with parsley sprigs, if desired.