Unless you've already acquired a certain taste for the comic originality of SCTV regulars Dave Thomas and Rick Moranis, you may want to pass up their new movie, "Strange Brew," now at area theaters.

Encountered unawares in this amiably silly but makeshift, hit-or-miss movie debut, their teamwork could provoke bafflement and hostility among total strangers--a sensation of having been overcharged for the purchase of a dubiously distilled or outrageously trifling home brew.

My own fondness for Thomas and Moranis didn't extend too far in the direction of the stoogey McKenzie Brothers, the characters used as a springboard for their aspiring and, I hope, successful movie careers. On the SCTV series, which purports to depict the programming and backstage crises of the most retrograde television network in North America, the inseparable, inane McKenzies--Thomas as Doug and Moranis as kid brother Bob--preside on a talk show called "The Great White North," which specializes in beer-guzzling and stupefying sibling prattle.

The McKenzies, caricatures of Young Canadian Manhood at its dumbest and lumpiest, embody a lowest common denominator that often seems more nerveracking than amusing. The illusion that televised conversation has finally sunk as far as it's likely to go is almost too convincing to be endured.

Since I'm much keener on Thomas and Moranis when, for example, they elaborate their sensational impressions of Bob Hope and Woody Allen, respectively, I was pleasantly surprised that they squeezed even a precarious feature out of the McKenzies. If anything, the characters seem better sustained in "Strange Brew" than they are in the typical SCTV skit. Not that the screenplay, written by Thomas and Moranis in collaboration with Steven De Jarnatt, could be mistaken for a masterpiece of humorous contrivance; on the contrary, it suggests hastily scratched notes assembled with squirts of Elmer's glue. The revelation is that the McKenzies' motley looks and personalities jell on the big screen, too, and better than I expected.

Doug & Bob remain the beer drinkers' answer to Cheech & Chong--similarly oblivious and indestructible, though anesthetized by suds instead of dope. But in addition to this fresh slant on the comedy of cruising through life in a permanent daze, Thomas and Moranis offer a distinctive blend of family psychology.

The essence of the McKenzie Brothers is that they've never outgrown their childish rivalry. They're still squabbling, teasing, pushing-and-shoving siblings. This element comes into focus with sometimes hilarious clarity in the movie, a point that may make "Strange Brew" peculiarly zesty to younger audiences. Unlike several of the summer comedies that jeopardized fundamental kid appeal by swearing themselves into "R" ratings, "Strange Brew" is unassuming enough to approach its likeliest public with an available and defensible "PG."

Thomas and Moranis suggest that the McKenzies never had much of a chance to elude ignorance, now identified as a genetic heritage. Mom and Pop McKenzie are introduced as a set of surly talking heads visible only from the back, their eyes glued to a cartoon telecast. The strangest single comic inspiration is the moment the boys accidentally barge in on a Primal Scene, and we discover the costars themselves doubling as the annoyed parents.

I suspect that a systematically weirded-out family farce, perhaps an updated version of the sort of family circle satirized by W.C. Fields in "The Fatal Glass of Beer," might have taken Thomas and Moranis further than the half-baked horror spoof plot they ultimately rely on. The cockamamie McKenzies live on a block that could be easily transposed to the creepy environment of cartoon domesticity in Joe Dante's amazing episode for "Twilight Zone--The Movie." Even the constant thirst for beer seems to be a family affliction, so pervasive that the pet dog, Hosehead, is a prodigious guzzler and the brothers appear to survive on a diet of beer and donuts.

In fact, the affliction must be catching, since the movie opens with the sound of Leo the Lion venting a beery belch. Starting on a playful, disarming note that could backfire if the film flops, Thomas and Moranis introduce their characters in the midst of flopping in a movie-within-the-movie. Supposedly, Doug and Bob so offend an audience lured to their science-fiction cheapie, "The Mutants From 2051 A.D.," which looks almost as amateurish as "Metalstorm," that they must hightail it for home, where their father immediately orders them out to pick up some brew.

Having squandered their father's beer money, the boys set out to con a free sample from the manufacturer, the Elsinore Brewing Co., and end up interfering with the mysteriously nefarious schemes of Paul Dooley as a crooked executive and Max Sydow as a mad scientist of a brewmaster who also supervises an adjacent loony bin.

Sharing directing duties on their first movie, Thomas and Moranis have had time to work out their own rapport and the sort of situations that might lend themselves to comic exaggeration given the McKenzie patterns of behavior. But they betray their inexperience in the scenes without Doug and Bob.

Neither triumph nor fiasco, "Strange Brew" leaves plenty of room for improvement, but I hope Thomas and Moranis get the chance to demonstrate that they've learned a lot from the mixed assortment of nuttiness in their first movie comedy. STRANGE BREW

Directed by Dave Thomas and Rick Moranis; screenplay by Rick Moranis, Dave Thomas and Steven De Jarnatt; director of photography, Steven Poster; production manager, Marc Dassas; edited by Patrick McMahon; music by Charles Fox; produced by Louis M. Silverstein. Presented by MGM/UA Entertainment Co. Rated PG. THE CAST Doug McKenzie . . . . Dave Thomas Bob McKenzie . . . . Rick Moranis Brewmeister Smith . . . . Max Von Sydow Claude Elsinore . . . . Lynne Griffin Jean LaRose . . . . Angus MacInnes