over charging

I'm working in a 90 w 250 for a friend it was over charging to about 18 volts or a little more , by cleaning connections i got it down to 14.8 to 15 v it's about a volt higher than i would like would an adjustable regulator be able to get it down around 13.6 or so . or is 14.9 - 15 volts alright . Thanks .