A knuckleheaded but amiable summer trifle, "Stroker Ace" is aimed straight at Burt Reynolds' vast heartland public.

In this instance, the fans get a lightweight series of comedy interludes and colorful background footage based on the theme of Reynolds as a roguish driving champion, Stroker Ace.

Between races in Daytona, Darlington, Atlanta, Talladega and Charlotte, the hero schemes to break his contract with an aggravating sponsor--Ned Beatty as the obnoxious founder of Clyde Torkle's Chicken Pit fast-food franchises--while ingratiating himself with the patron's adorable publicity director, Pembrooke Feeny. This is an absurdly contrived film debut for the gorgeous bombshell comedian Loni Anderson.

The heroine and the race tracks are the movie's preeminent pictorial attractions, and both acquire awesome, mythic dimensions on the big screen that are evidently obscured on television. Although no one seems to bother about filtering an excess of haze and glare out of the track footage, the panoramic views of these huge sporting arenas give you an appreciation of their sheer size and sensuous overload of teeming humanity and roaring machinery. As for Anderson, she looks rapturously unreal on the movie screen.

Her lustrous cascade of platinum hair, with never a strand out of place, and her rosily lacquered visage are aided by an innate likability and disarming skillfulness. Her soft voice and minty-fresh diction combine for a distinctively hypnotic sound, and it becomes quickly, delightfully apparent that her comic timing is potentially more devastating than her looks.

Nevertheless, the key miscalculation in "Stroker Ace" is keeping the romantic comedy aspects of the material subordinated to both the racing sequences and the farcical enmity between Reynolds and Beatty. In addition, the leading roles themselves have a moronic tone, allowed to prevail by director Hal Needham and his screenwriting partner, Hugh Wilson. I could imagine Reynolds and Anderson being a memorable kick together, but they'd have to be playing characters upgraded from tongue-in-cheek infantilism. There's really no reason why performers this grown-up, capable and already popular should be acting out a courtship ritual at the "Grease" level.

The megaflop of last summer's "Megaforce" seems to have stirred Needham to tidy up his directing act to some extent. He still insists on illustrating the end credits facetiously with outtakes, but the galloping ineptitude of his most recent features--"Megaforce," "The Cannonball Run" and "Smokey and the Bandit II"--has been reduced to a tolerable trot.

The best of the funnier bits finds Jim Nabors, cast as Stroker's head mechanic, Lugs, trying to straighten out his features while singing "Amazing Grace" after Reynolds complains that his powerful singing voice is curiously undermined by a facial peculiarity: the mouth won't stay in place under the nose and keeps sliding off to the sides. John Byner gets a chance to toss off his Johnny Mathis vocal impression when first introduced as Stroker's old boyhood pal Doc, but the script isn't calculated to follow up on his comic virtuosity. Bubba Smith lends his imposing humorous presence to the role of Clyde Torkle's chauffeur, but he and Beatty don't really function as an oddly matched comedy team, as Pat McCormick and Paul Williams did in "Smokey and the Bandit." STROKER ACE

Directed by Hal Needham; screenplay by Hugh Wilson and Hall Needham, based on the novel "Stand On It" by William Neely and Robert K. Ottum; director of photography, Nick McLean; art director, Paul Peters; edited by Carl Kress and William Gordean; music by Al Capps; produced by Hank Moonjean. A Universal Pictures and Warner Bros. presentation. Running time: 96 minutes. Rating: PG. THE CAST Stroker Ace....Burt Reynolds Clyde Torkle....Ned Beatty Lugs....Jim Nabors Aubrey James....Parker Stevenson Pembrook Feeney....Loni Anderson