The Prize Fighter(1979)

"Bags" the boxer (Tim Conway) and his manager, Shake (Don Knotts), are quite a pair: One is a dim bulb, and the other has a mean streak. Times are tough and they must save their gym, so they line up some money making fights. But when Bags and Shake discover that the bouts have been rigged, they end up with their backs to the wall and must fight back -- literally.