To do a harley. Is to vibrate ones lips on the clitoris and make a vibrating, V-twin engine sound, that not only entertains the recipient of the cunilingis, but also acts to vibrate and arose the said area.
Girl1: Oh my god last night this guy did a harley on me.
Girl2: Really, was it good?
Girl1: Yeh it was amazing, but it was loud and my dad thought there was a motorbike in my room.
Girl2: I really want one, will you show me?
Guy1: I did a harley on a girl last night and she was wetter than an otter's pocket.