What type of generator do you have, Make, Model, Year
Some generators rely strictly on engine RPM to control the output, volts at 120 and hertz 60. If the RPM's are too high or low it will result in higher or lower volts and hertz. The engines will have governors to regulate the RPM's which should be preset.
Newer generators use an inverter type systems which provide a constant 120 volt and 60 hertz. My Onan 7.5 diesel is like this.
If you are seeing over voltage it may be that your governor isn't properly regulating the RPM's.
2002 Monaco Windsor 38 PKD
2005 Jeep Grand Cherokee w/5.7 Hemi