Past is prologue in “The Conjuring 2,” and it’s the most effective sequence in James Wan’s movie. The sequel to 2013’s surprise hit opens with the infamous paranormal investigation that made Ed and Lorraine Warren household names, if those households subscribed to the National Enquirer.

The name says it all: Amityville, in an allegedly haunted house that spawned 14 horror flicks, mostly rubbish. Wan stages a chilling re-enactment of the house’s factual mojo-spoiling event, in the context of a seance shaking the Warrens’ resolve to bust ghosts. The passage thumps with dread, and suddenly a 15th version of Amityville’s horror would be just fine.

Then “The Conjuring 2” gets down to new supernatural business that never quite measures up to its prologue or preceding movie. Wan still knows how to tighten screws and visually and sonically stab our nerves. As the Warrens, Vera Farmiga and St. Petersburg’s Patrick Wilson maintain a warm, winking bond in the face of demons.

Yet the haunting chosen from the Warrens’ trophy case of supernatural souvenirs isn’t deserving. Basically it’s an exorcism gig with working class British accents. Peggy Hodgson (Frances O’Connor) is a single mum in a London flat with four children, one demonically possessed. That would be Janet (Madison Wolfe), a bright child when she isn’t snarling.

With the original “The Conjuring” and “Insidious,” Wan led a revival of old school horror values, visceral without being explicit. Now that those experiments in terror are bona fide franchises, he’s toying with the retro format, not always successfully. “The Conjuring 2” runs nearly 2 1/4 hours, uncommon for the genre, and a signal of its ambition.

One uneasy example: Well into the possession stuff, a phonograph fails when the Hodgson family could use some music. Ed picks up a guitar, and Wilson does a decent Elvis impression, singing “I Can’t Help Falling in Love With You” — all the way through. The scene ends, and viewers may wonder why it’s there. Me too.

Here’s what I think: Wan wants us to think something frightful is about to happen, that Wilson’s soothing voice will be suddenly interrupted by something REALLY SCARY. Then it isn’t, and Wan thinks we’ll laugh away tension, that really wasn’t built. Nice idea, with slightly off execution. At least Wan tries something different.

Not always, though. At times, “The Conjuring 2” takes a MadLibs approach to terror, subbing a demonic nun and crotchety old corpse for what-have-you in the nightmare collection. Janet’s possession is one head swivel and a vomit spray shy of Linda Blair’s version. Wan is gradually remaking cliches of the cliches he resurrected to make two franchises possible.

“The Conjuring 2” is serviceable horror, heavy on the audio stings yet smarter than the average gorefest. That’ll be enough to justify a Part 3, in which case I’d like to show Mr. Wan an interesting property in New York. Little town called Amityville …