Well different torrents will have different speeds depending on how well seeded it is, what i do which keeps my dL rate consistent (with well seeded torrents) is limit the upload speed to about 10 kb/s and keep you're download speed unlimited. You'll find these setting under bandwith allocation when you right-click a new torrent, also set the bandwith allocation to high.
50 kb/s is awfully slow what kind of internet connection do you have?