T.J. and the Pirate Who Wouldn't Go Home (Vintage Paperback) by Carol Gorman

Captain Billy has to go back to the past! 

T.J.'s eccentric Uncle Ainsley builds a time machine that accidently brings a pirate from 1690 to the present, and the pirate, Captain Billy, is having such a good time in the modern world that he refuses to return to his own time.