How loose is it ? Very or just a bit slack . That is the first thing to determine . Is the gun well on face and reasonably tight in the body? Dose the lever have any play on its spidle when the retaining pin is removed ?Are the barrels hard on the flats when the lever is closed ?
Sorry about all the questions but without this it is difficult to suggest the simplest way to remedy your problem . I don't think any one who built guns of this period thought that over a century later that anyone would still be using them so the idea of correcting for wear never crossed their minds . The Jones lever is one of the more difficult systems to repair and can be an absolute pain ,particularly if they are badly worn . Not the easiest of jobs at any time .
