Gains 10 mins per hour?

Carriage clock. Donated to charity shop where I help. Very dirty, balance spring not connect. Small bit of spring found. Cleaned it out connect balance spring with as small as possible section, about 1mm in fixing post, if that. But even with adjuster fully over gain dramatic. I think, that someone, before it was donated messed on with it, and possibly shortened the balance spring by a turn. That is all I can think of. Any one any other thoughts, also if it is balance spring, are they available and where. Thanks.