Carbon on the valve stems can give you squirley performance. I bought an old os.91fs several years back. Ran it for about a year then set it aside. When I came back to it, it didnt want to idle very well. It had some carbon on it but I didnt think it was that bad. Cleaned it anyway and what didnt come off in the crockpot got ground off with a crown wire brush in a dremil. I removed the valves and clean them up too so it was as clean as you could get. What a difference! Runs like a top and idles lower than my saito's. Dont know if that could have anything to do with your valve springs breaking though, anybody else chime in (W8ye)?
Edwin