Request #647 From Joy

I read a book in the late 80s, early 90s, but I’m pretty sure it was published 20 or 30 years earlier.. It was definitely set in the 60s (or maybe 70s?) It was set in Scotland. Main character is a teen named Moira. She dates a guy with a motorcycle, much to the disdain of her strict father. I think she leaves home because of that. She has a younger sister. I want to say the cover is a photo that shows the guy with his motorcycle and the girl in  a very short skirt.

Thanks for any help!