I have a Craftsmen 3.7 C.I with a 24" bar that saids 40:1 and after burning it up and having rebuilt for $550.00 I now run it at 32:1 with no problem.
If you think it past 30 years old i would try it out at 25:1 to see how how much smoking it dose, then move up to 32:1. but would not go to 40:1.
Richard, May 2008