I disagree.
All mechanically-coupled, direct engine-driven alternator heads produce a true sine wave AC output waveform. In fact, it would be tough to design such a direct driven alternator head to produce anything else. It appears the OP's generator is a mechanically-coupled direct engine driven alternator.
Wrooster
Sine wave in theory. Most small generators produce power with 5-8% harmonic distortion. This is due to the design and materials used. As a result the output is not quite a clean sine wave. Fine for nearly all loads, but not quite a "true" sine wave.
paul