If Jims suggestion doesn,t work its as he says replace it ,or you can overhaul the Gen. It,s a time consuming job but what also can be done with raised mica/insulation between segments is to get a hack saw blade, grind one end into a hook shape a bit thinner than the insulation, sharpen the hook and use it to rake out the excess insulation. If the Gen is that old though it would also be worth getting commutator skimmed. They are a bit like brake drums, can get out of round over the years. Used to be done years ago like that, very time consuming. It,ll cost a bit for someone to do it or it,ll cost you a few hours. Might be worth checking brushes are sliding easily in brush holder. If the comm is out of round and brushes are a bit tight they,ll only wipe the commutator on the high spots. John