The experience of sitting through “Dumb and Dumber To,” which reunites stars Jim Carrey and Jeff Daniels with directors Bobby and Peter Farrelly 20 years after their maiden voyage to the lower depths of imbecility, is something like watching an aging MLB slugger wind down his career in the minor leagues. The stakes are far lower, he’s slow off the blocks, his joints are creaky, and his batting average is a pale shadow of what it once was, but the swing and the stance still look the same, and you can’t accuse him of not suiting up. Sporadically very funny, mostly very tedious, and sometimes truly vile, this 18-years-too-late sequel nonetheless exhibits a certain puerile purity of purpose, and should accrue healthy profits playing to the nostalgia of the dumb and the dumb at heart.

At the risk of applying rose-colored glasses to reconsider a film that aspired to be nothing more than a lobotomized Three Stooges routine, the original “Dumb and Dumber” possessed a certain je ne sais quoi that distinguished it from its fellow bottom-feeders. Featuring Carrey at the height of his rubbery powers, and a pair of first-time filmmakers who were very clearly learning on the job, the combination produced a strange sort of fever-dream atmosphere that allowed audiences to laugh at things they knew they probably shouldn’t. Though all involved are now old enough to know better, a bit of that anarchic spirit remains, and the sense of sad obligation that hampers most similar sequels is little in evidence here. Carrey and Daniels certainly appear to be having more fun than most viewers will, with Daniels in particular relishing the opportunity to shake off the stern shackles of his role on “The Newsroom” and let the fart jokes fly.

Lest this reviewer get too misty-eyed describing a film whose first five minutes feature a cat named Butthole eating crystal meth and a man being dragged to and fro by his catheter, it must be said that the vast majority of the gags in “Dumb and Dumber To” sputter out. Aggressive attempts to create quotable new routines lack much spark, and lame callbacks to the first film (“Want to hear the second most annoying sound in the world?”) take up entirely too much space. In truth, very few of the jokes here are much sharper than what one would find in a water-treading latter-day Adam Sandler project; there’s simply a lot more of them, delivered with greater conviction.

The plot, such as it is, is kicked in motion when Harry (Daniels) enlists Lloyd (Carrey) to help him find a kidney donor. The two quickly discover that Harry once sired a love-child with neighborhood slattern Fraida (a game, poorly served Kathleen Turner). Now in her 20s, Penny (Rachel Melvin) was adopted by famed scientist Dr. Pinchlow (Steve Tom), and her behavior leaves little doubt where the Farrellys come down in the nature-vs.-nurture debate.

Before Harry and Lloyd can intercept her at home, however, she’s sent off to deliver a keynote speech at a TED-style conference in El Paso, while the good doctor’s trophy wife (Laurie Holden) and her handyman heavy (Rob Riggle) plot to kill him and make off with his riches. Like the original, most of “Dumb and Dumber To” takes place during a long road trip to reach her, as our heroes are pursued by a criminal conspiracy that they are utterly ill equipped to recognize.

Carrey expends substantial energy trying to recapture his manic comic energy of old, before his more sincere roles in “Eternal Sunshine of the Spotless Mind” and “The Majestic” left his career in a neither-nor limbo, and he largely succeeds through sheer effort alone. His face has lost little of its elasticity, and the sight of him messily devouring phallic-shaped foodstuffs will trigger viewers’ gag reflexes with ruthless precision. In contrast with such lovable oafs as Seth Rogen and Danny McBride, who have supplanted him as cinema’s man-children du jour, Carrey’s comic instincts still tend toward the sinister, and many of this film’s jokes live or die depending on which side of the cruel-clever divide they fall.

With six credited screenwriters — a credit that initially reads as a meta joke — the film can’t help but stumble upon a few choice lines (“It’s all water under the fridge,” “you’re deaf as a bat!”), even if one must wade through a lot of guff to find them. Technical credits are rough and ramshackle, but if you wandered into “Dumb and Dumber To” to study the cinematography, the joke is clearly on you.