"The Tim Conway Show" opens with its star sitting on the lap of a gigantic ventriloquist's dummy, and at the sight of it, all kinds of delightful comic possibilities -- including a well-deserved lampoon of that empty movie "Magic" -- come to mind. Unfortunately, they didn't come to Conway's mind, nor that of his co-writer, Roger Beatty, and so the dummy remains a neglected hilarious prop.

Imagine having a great big dummy like that and not doing anything with it.

Though only sporadically and never spectacularly funny, the CBS special, at 8 on Channel 9, does make one wistful for the days when the networks had room in their schedules for classy comedy-variety hours. The memory is made more graphic by the presence of Carol Burnett as a guest star and by the fact that the Conway hour was produced, as was Burnett's own series, by her husband Joe Hamilton.

Burnett, looking better than ever, has a dance number with a crowd of tots near the middle of the show that, while it doesn't knock you out of your chair, is so bright and jolly that you realize how little else on television is these days. No one is keeping TV's weekly variety-hour traditions alive, and it's as if vaudeville had died all over again.

The Conway comedy sketches range from so-so to iffy. First, he returns as Mr. Tudball, from the old Burnett show, though now he is Inspector Tudball -- a cross between Clouseau and Columbo, basically -- and Burnett as Mrs. Wiggins is involved in a pointless mock-murder mystery.

With guest Don Knotts, another highly welcome presence, Conway is the host of an accident-prone TV cooking show; with Burnett he plays a divorced husband who finds his under-alimonied ex-wife is the maid in his $300-a-day hotel suite; and, in the closing sketch, Conway again plays his familiar slow-motion little old man, this time as the proprietor of a "fully automated" post office.

The comedy writing is by turns weak or labored, and so steadfast in its innocuousness that one longs for a nasty or disparaging word.Conway, whose friendly Elmer Fuddy features are a living invitation to laughter, has clearly underextended himself.

The special does, however, answer the semimusical question, "On which show will Village People appear tonight?" This one -- the gaily attired disco group materializes in a drained swimming pool singing "YMCA." The entertaining thing about Village People's decreasingly outrageous act is that it manages to look incongruous in any setting -- perpetually and amusingly out of place.