Trim any thick fat from the outside of the pork butt. Cut the meat into 1-inch cubes. Discard any cubes that are pure fat, but don't try to trim all of the fat out of the meat.
Put the pork in a large pot. Add the water the strips of orange zest, garlic, onion, red pepper flakes, cinnamon, bay leaves, oregano, 1 1/2 teaspoons salt and cloves. Bring to a boil then reduce to a simmer. Skim off any scum that forms on the surface. Simmer for 1 1/2 hours, until the pork is very soft, adding more water if necessary to keep the meat submerged. Season with salt.
Bring to a gentle boil and cook until the water has evaporated about 30 minutes. Cook a little longer to fry the meat slightly. (Note: if you want extra-crispy carnitas, fry approximately 25 minutes.) Watch carefully to prevent burning.
Remove the bay leaves and cinnamon sticks. Put the meat in a fine strainer and push gently with a wooden spoon to remove any excess fat. Discard the fat.
Fold a few tablespoons of carnitas inside each tortilla and top each taco with cilantro, onion and salsa.