It is possible that the generator is not intended for “smooth” motor starting. The inrush current for most single-phase motors is usually of poor power factor and much greater than the steady-state {nameplate} value.
I would try loading the generator with a purely resistive load—like incandescent lamps—piecemeal approaching the rated load a little at a time. A voltmeter should reflect what you may also see in lamp brightness. That may get you started on resolving the problem. I would not invest in an outboard voltage regulator like a Sola transformer—the results will probably be disappointing. A larger generator, purchased with a guarantee that it will have adequate capacity—may be the only effective solution.
