1994 Ford F-150 won't start after rain

If it's raining outside and if my truck sits for 1 day without being driven it takes forever to start and sometimes won't start, I just have to let it set until it warms up enough outside to dry it out. In the old days we put a felt liner ring in the distributer cap I'm wondering if thats what this was for, new trucks don't seem to have these. Maybe it's not the D cap at all I don't know