As an event, the Beach Boys' free concert on the Mall Saturday was a success. It drew a Woodstock-size crowd of 400,000 and the morning rain provided Woodstock-style mud to splash in. When the Beach Boys hit the gorgeous opening chords to "California Girls" at 6 p.m., the sun broke through the clouds for the first time all day. The ensuing songs celebrated fast cars, sunny days and true friends with a distinctive American brand of optimism.

As a musical performance, however, the show pointed out the band's continuing artistic dilemma. Though the group has recorded a wealth of strong material since 1968 songs Saturday. This unnecessary emphasis on nostalgia caused founding member Carl Wilson to quit this summer's tour in protest. Dennis Wilson, Carl's older brother, has rejoined the tour after similar protests in the past, but declined to sing Saturday. Brian Wilson, the oldest brother and composer of every famous Beach Boys song, was little more than a prop as he played piano chords and sometimes sang embarrassingly off key.

This left the show in the hands of Al Jardine, Mike Love and Bruce Johnston, who seemed to have little feel for a great musical legacy. They clowned around and undermined beautiful songs like "Wouldn't It Be Nice" with sloppy arrangements. They also passed over a long list of superlative songs to sing such weak selections as "Long Tall Texan." Despite all these drawbacks, the stubborn strength of the old songs did shine through, as when Brian Wilson overcame his notorious stage fright long enough to sing "God Only Knows" with all the haunted longing the song deserves.