Even as a tenth-rate "east of Eden" spiced up with themes and variations from "Rocky" and "The Champ," tonight's premiere of "The Contender" on CBS strikes an awfully purple pose. It is corny beyond belief, sentimentality but it's also idiotically eventiful enough to be tolerably entertaining.

Marc Singer plays an esoteric Oregon prizefighter who struggles to escape from Palookaville in this 90-minute film, at 10 on Channel 9, that tests the waters for a new series to continue next week. It's one of those shows with little to recommend it other than the fact that whatever else is on at the same hour could very well be worse.

The three writers and two directors who patched this crazy quilt together throw all they've got at the poor hero, and a lot of what they've seen in other lousy movies. One minute he hops into the sack with a pretty schoolteacher and the next he hops into her classroom to quote a little Shelley.

A bozo at the gym picks a fight with him over his love affair, his little brother is beaten up by a sadistically prankish cop, his father gets bopped by a runaway log at the old lumber mill, the girlfriend demands "a clarification of this relationship," and as if that weren't enough, a now-crippled politely asks sonny-boy if he'd mind getting "the 12-gauge" out of the attic so Dad can shoot himself to death.

"I can't do it, Pop," Singer says.

Soon we move to Fresno where the fighter strikes up a friendship with an unbearably sassy young teen-age girl (Tina Andrews) given to such unconvincing outbursts of alleged streetargot as, "I'm gonna rip your arm off at the elbow."

This unforgivable black stereotyping is almost topped by the casting of Moses Gunn as the rumpled old cuss who volunteers to manage The Kid. Gunn, an actor of considerable resources, is stuck with lines like "Atta boy, Johnny!" and "I got me the great white hope."

In fact, the writers have scored a dubious victory in that there isn't one moment of this half-crocked potboiler that doesn't seem vengefully familiar. The knockout punches come in a climactic exchange between the reconciled lovers after The Big Fight; it brings to mind the catcall of a spectator depicted in the hilarious satire "Singin' in the Rain": "Did somebody get PAID for writing this dialogue?"

He: "When I come back, things'll be right between us, 'cause I'll feel right about myself." She: "Johnny, I know what you have to do. You're right. You gotta go all the way." Quick somebody throw in the crying towel, puhleeze!