The fast-food restaurant operator said its US business is in the doldrums as Taco Bell, its most profitable brand in the United States, struggles to recover from bad publicity surrounding a dropped lawsuit over the beef content of its taco filling. One day after the owner of Pizza Hut, KFC, and Taco Bell reported a 10 percent rise in quarterly profit, driven by surging sales in China, the company said it was bracing for more US declines. But investors shrugged off the domestic struggles. Louisville, Ky.-based Yum generates nearly three-fourths of its operating profit from abroad. Yum also operates Long John Silver and A&W but has put those chains up for sale.