capacitive loads will increase the terminal volts, inductive reduces them.
I don't know off hand the orders of magnitude, but you might see 1 volt change in terminal volts per amp of reactive current.
the last generator i had, put out 260 volts no load, 240 under 6KW load, 60Hz for both.
it wouldn't take much to make it output 270 volts under no load, but under load it would quickly drop.
getting a small generator to 270 under full load certainly sounds like a regulator or governor problem.
you could have something interesting going on if the generator is a third harmonic resonant excited type.
you'll recognize these by a diode and capacitor located on the rotor, and there are no brushes.