We are getting 155v on the 120 v receptacles
it may be running far to fast, just slow it down a little
how did you get this reading,,,on load or off load???
if off load hook up a light bulb and take another reading,,if its pulled the volts back down thats fine,,,if not plug in to the geny some thing thats close to its full load,,,"not more than this" now take the reading,,,
if still high lower the reves till its just 120v on full load
mind you,,,5000watts? thats a big un!!!!