THERE IS A PLAY somewhere in ''Park Your Car in Harvard Yard,'' but the director and the producers will have do some additional work to find it.
They might begin with the length of the play, one that opened last evening at the Mechanic. At present, the comedy-drama runs for some two hours and 35 minutes. That's about average for most plays, but in this case, it is far too much running time. Tightening is in order.
''Park Your Car in Harvard Yard,'' set in Gloucester, Mass., is a long time getting to the plot. A certain amount of this preamble is welcome, even necessary, but in this instance, we get too much. The plot doesn't begin to form until the first act is almost finished, and then for much of the second act, it is ignored. As the play moves toward close, the basic plot resurfaces, and when it does, the production comes to life.
''Park Your Car in Harvard Yard'' certainly has potential. Written by Israel Horovitz, it is about an 84-year-old high-school teacher (music appreciation and English literature) who has seriously affected the lives of some of his students and not always for the better.
This is an unusual, even novel, theme, one that touches just about everyone. Most people can remember at least one high-school instructor they would like to see linger in hell. ''Harvard Yard'' is on the right track, but it is much too slow getting to its destination.
Jason Robards plays the teacher. He may be in need of added rehearsal. Last evening, he seemed to be playing for time, searching for words. An 84-year-old man might do this, but he probably would not grope as much as this man does. There is a performance here. All Robards has to do is find it.
Judith Ivey co-stars. She is Kathleen, an Irish girl who is undereducated but world smart. She, like her mother, had been given a failing grade by Jacob Brackish (Robards), who had done the same to the man Kathleen married.
Kathleen may be laying a little too much on the man, by blaming him for all her family's unhappiness, but there is more than just passing logic in what she says.
Ivey does extremely well as the live-in housekeeper. Her motivation is a little slow in coming, but this, too, can be changed with a bit of rewrite.
Horovitz and director Zoe Caldwell might also eliminate the business wherein the girl slaps herself during an argument with the old man. It is more disturbing than it is dramatic, and you do have the feeling that all this hatred might be better directed against the old man, who deserves it far more than the girl.
Horovitz has written some very funny plays, and there is humor in this one, humor that will play better when the script is pruned.
The music, in the beginning, might also be toned down. Classical in selection (when the teacher is at the console), it is, at times, intrusive. At others, it adds to the enjoyment of the evening. It's a simple matter of modulation.
''Park Your Car in Harvard Yard'' will remain at the Mechanic through Oct. 27 then head for New York. Though the author has written many plays, this will be only his second to reach Broadway. With revision, it could make it.
''Park Your Car in Harvard Yard'' ** A retired high-school teacher hires a live-in housekeeper who has more than dusting and cooking on her mind.
CAST: Jason Robards, Judith Ivey
DIRECTOR: Zoe Caldwell
L RUNNING TIME: Two hours and 35 minutes with one intermission
! TICKETS: 625-1400