Trim any thick fat from the pork butt. Cut the meat into 1-inch cubes, discarding any that are pure fat, but don't try and trim away all the fat in the meat, that's needed for flavor and to help with browning later!!
Put the pork cubes in a large pot and cover with water. Bring to a boil and then reduce to a simmer. Skim off any scum that forms on the surface during the first 15 minutes. After scum is skimmed off, add the chopped onion, garlic, orange zest, crushed red pepper, cinnamon, bay leaves, oregano, cloves and salt. Simmer uncovered for another 1¼ hours until pork is very tender, adding more water if necessary to keep it submerged. Season with more salt. (I add the aromatics after removing most of the surface scum because otherwise, too much of it gets skimmed out with the scum.)
Continue simmering at a gentle boil until the water has evaporated, about 30 more minutes. Remove bay leaves and cinnamon. Cook a little longer to fry the meat, stirring often and adding a little water it it seems to be sticking and/or burning. (Alternately, at this point you could transfer the meat to a nonstick skillet to fry it longer for crispier meat.)
Spoon some carnitas onto each tortilla. Top tacos with chopped onion, cilantro and salsa. Serve.