It may be asking too much, even of a comedian, that he come up with scads of riotous stuff while lying on his back recovering from a heart attack. Still, the mirth doled out by George Carlin in his third and latest Home Box Office special, "Carlin at Carnegie," will seem awfully familiar, and not always cheerfully familiar, to those who've faithfully followed Carlin's troubled career, one punctuated with as many crises as Nixon's.

A gifted and inventive comic observer of certain small pockets of life on this planet, Carlin, at 45, suffered his second heart attack in May and recorded this HBO special at Carnegie Hall Oct. 10; its first HBO showing is tonight at 8. The orchestra chairs are piled behind Carlin on the stage of the great hall, giving the impression that he and a full house of laughing fans sneaked into the building, while none of the authorities were looking, and reveled in irreverence.

The indications are, though, that Carlin spent a bit too much of his recuperation staring into his refrigerator or contemplating bowls of Rice Krispies. His meditations on, and physical impressions of, various foods grow rather tiresome. One of Carlin's mock newscasts is more successful, and his reflections, later, on the privileges enjoyed by dogs and cats are quite funny, if also not entirely new.

It's when Carlin is his most scatalogical that he's at his best. The lack of censorship in pay TV allows Carlin not only to repeat the original "Seven Dirty Words" that caused such havoc when New York radio station WBAI-FM aired the routine in 1973, but to amend the list lavishly and amusingly as the one-man show is winding down--actually, revving up.

Carlin's humor is not only verbal but physical. He convincingly turns himself into animals, vegetables, perhaps even a mineral or two, and he talks, briefly, about his heart attack and how he appears to be continually racing Richard Pryor to the hospital (this line of thought could be developed into material as self-effacing, and painfully funny, as Pryor's recollections of his night as a human torch).

The special, produced by Carlin's wife Brenda and nimbly directed by Steven J. Santos, opens with Carlin asking Harlemites how to get to Carnegie Hall and getting very New Yorkish answers. It closes with Carlin telling the crowd, poignantly perhaps, "Good night--and I hope I see you another time." Despite the recycled material, the hour has to be counted as a success because it seems much shorter than it is. Carlin has not lost his touch, and his touch is frequently cherishable.