I have had such triggers adjusted by a competent gunsmith. In both cases, single and set. A trick I have used in old (and new) triggers is to clean them thoroughly with action cleaner or brake cleaner and then lubricate them with a high quality synthetic gun oil. I have used Rem Oil and Break Free with good results. In fact I disassemble the entire action and give it a good dousing with the lubricant. I remove all the excess and reassemble. It never hurts, even in the best of cases. In the worst of cases, it smartens them up.