Brown the ground beef in a large skillet over medium-high heat until it is no longer pink; drain. Add the onion, bell pepper, chipotle chilies, adobo sauce, flour, cumin and salt to the beef. Cook for 5 minutes or until the vegetables are tender, stirring frequently. Add 1/2 cup of the sour cream and stir until mixed.
Meanwhile, heat the taco shells as directed on package. For each taco, spoon 1/4 cup beef mixture into each taco shell.
Top each taco with cheese, lettuce, tomato and a dollop of the remaining sour cream.