Comedy is all about timing. The dimwitted Lloyd (Jim Carrey) reminds the audience of that simple fact minutes into “Dumb and Dumber To” and the sentiment echoes throughout the disappointing return. Twenty years have passed since audiences first met and improbably fell for Lloyd and Harry (Jeff Daniels), and the boys are up to their same old stunts, which is the fundamental problem: They’ve stayed the same. We’ve changed.

“Dumb and Dumber” was received favorably enough by critics when it was released in late 1994 — back when Jim Carrey seemed like he had the potential to be the next Jerry Lewis. But no one could have foreseen the effect this ridiculous tale of a couple of idiots traveling cross country to return a briefcase would have on the culture in the coming decades.

Lloyd and Harry’s antics and one-off lines penetrated our collective imaginations and managed to do that thing that all comedies dream of — get better, or at least more beloved, with time. The ill-conceived sequel, however, is so uninspired that it could retroactively tarnish our affection for the original.

This next chapter technically picks up where we left off, at least in terms of Lloyd and Harry’s relationship. Lloyd has been pretending to be comatose for 20 years, all for the sake of a gotcha moment, which Harry fully enjoys.

Anyway, Harry soon reveals that he’s in dire need of a new kidney. The story is set in motion when he discovers he might be the father of a daughter, Penny (Rachel Melvin), and they head off to find her. Plot is almost beside the point, but it does follow the beats of the first to a tee. There are road trips set to jaunty songs, straight men to annoy and a rich woman with nefarious plans that they’ll inevitably screw up. Lloyd even has a new pretty young thing to dream about in Penny.

But, what may have been subversive and irreverent in 1994, now just seems vulgar, hateful and tone-deaf — from their unabashedly misogynistic treatment of women (including Kathleen Turner as Penny’s mom) to their insidious racism. In the harsh light of 2014, their juvenile buffoonery looks embarrassing and lazy. It’s hard to imagine a new generation latching on to this brand of humor.

That’s not to say that the movie is entirely without merit, or laughs. Co-directors Bobby and Peter Farrelly adore their characters and are trying to give fans what they want: the nostalgic joy of an unfussy reunion, devoid of modern snark or meta commentary. Stars Carrey and Daniels are clearly giving it their all, too, as energetic and vital as ever with their strikingly aged visages. Carrey’s, in particular, has reached a peak of elastic malleability, giving him a grotesquely broad palette to play with.

Also, Rob Riggle’s entry mid-movie adds a much-needed dynamism to the film by finally giving the guys a suitable comedian to play against. He’s responsible for most of the very few laughs and a visual gag so brilliant that some might even be compelled to see it again. But ultimately, it’s too little, too late.

Some things are just better left in the past. It’s a shame “Dumb and Dumber” wasn’t one of them.

“Dumb and Dumber To,” a Universal Pictures release, is rated PG-13 by the Motion Picture Association of America for “crude and sexual humor, partial nudity, language and some drug references.” Running time: 110 minutes. One and a half stars out of four.

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MPAA rating definition for PG-13: Parents strongly cautioned. Some material may be inappropriate for children under 13.