Believe it or not, You're a Big Boy Now has some faults. It was written and directed by a 27-year-old new-comer with complete control of his film, it has a uniformly fine cast, and the Lovin' Spoonful sing the background music, but the movie is not a stunning click-your-heels-in-the-air success. It's just a fresh, fast, clever movie.

It tells the story of 19-year-old Bernard Chanticleer, son of the curator of pornography at the New York Public Library. In a hurry to achieve manhood in the approved way, young Bernard must choose between the love-sick squarehead who works with him and a sadistic nymphomaniac go-go dancer who doubles as a non-speaking actress in an off-Broadway play. In the end, of course, the right girl wins.

The movie's faults are those of excess--a little too much freshness, fastness, and cleverness at times. Tricky gimmicks are repeated too often or dwelled on too long the first time around. Possessive Momma sends her son locks of her hair; a hypocritically-pious spinster drags a burly cop off to bed; the bitch-goddess type has tacked above her bed the wooden leg of her first seducer--an albino hypnotherapist. It can be too much, even if you are prepared to accept most anything.

But if there must be excesses, let them be of talent. The young director, Francis Ford Coppola, generally exploits his medium well. For example, the hero is with plain-Jane but his mind is on bitch-goddess. How do you know? Because her name suddenly appears on the electrotape news around the Times Building and her go-go body lights up a neon sign.

The cast is pretty high-powered too. Momma is Geraldine Page (you might check today's page one to see if she has won an Academy Award), lecherous poppa is Rip Torn; lecherous spinster is Julie Harris; Wise-guy friend is Tony Bill; big boy himself is Peter Kastner. But strangest of all is Elizabeth Hartman--the blind girl you all knew and loved in A Patch of Blue--as the disco dancer. She shakes, she shimmies. she is the most unremittingly evil person you'll meet all week.