12.5 KW Kohler Generator: How do I increase AC voltage?

[The Newell]

The one on the drivers side will put out 120 volts.

The one on the passengers side only puts out 102 volts. When I increase the RPM's the voltage goes down.

Any thoughts? How does one go about increasing the AC voltage on a generator? I thought the speed increase would have helped.

BTW - I ran both AC units, turned on all of the 120v lights to see if the load would cause the Kohler to choke. It didn't miss a beat and kept putting out 102v. I only did this for 30 seconds as lower voltage will increase the amp draw of and could burn out an AC motor.