In older windows, the balancing weight is the large iron cylinder attached to the movable sash with ropes. Keep it or snip it?
Snip it. In most cases, there will be a sash replacement balancing mechanism to keep your new sash in place and properly moving up and down. Obviously if you are buying salvaged sashes from an architectural salvage store, there won't be any balancing mechanism. So, use your best judgment when deciding what to do about the sash weights.