first off,isn't a 3-71 a GM-Detroit engine? anyway,sounds like you need a new AVR(voltage regulator)make sure your insulation is ok on the gen,get an AVR with 3phase sensing,it allows tighter voltage regulation;you probably have an old silverstat AVR,just replace it with a new modern AVR(3hot...
try using a hot air heat gun on the potting material-
if it's a silicone based it will become pliable enuf to flake out w/a probe or screwdriver;it worked for me