Harmonics is the sound wave. It should look even on a graph or make a smooth feeling beat. It may have a different name at low rpm but...
One time a harmonic balancer fell off our car IN THE DRIVEWAY
even though the motor wasn't running fast it sounded awful. Like scary and klunky.
I think even a one cylinder motor like a lawn mower has some balance issues, otherwise they wouldn't put counter-weights on the crankshaft of a lawnmower engine.
Safety First Friends.
