"This is the first year that the Manassas, Jazz Festival didn't start on time -- that it wasn't my fault," confessed festival producer Johnson "Fat Cat" McRee last night at the Elks' Lodge in Manassas. Last minute adjustments to the array of recording equipment were holding up the commencement of the 15th consecutive year of the oldest continuous traditional jazz gathering anywhere.

But start it soon did with a roar as the Smith Street Stompers from New York tore into "Jazz Band Ball," rendering the familiar unfamiliar. In the words of one veteran listener in the audience of several hundred, "They're different . . . Swing like hell."

The roster of the festival, dedicated to the memory of the late Jelly Roll Morton -- pianist, composer and self-style "inventor" of jazz -- numbers nearly 40 musicians including a small corps of pianists fluent in Mortinesque phraseology. Among these are Dill Jones from Wales, Harvey Jacobson, and locals John Eaton, Larry Eanet and Al Stevens.

Longtime observer of the music, Lou Byers described the lineup this year as "a tremdenous variety of talent covering almost every aspect of traditional jazz."

Heading the list of those scheduled to perform are former big-band trumpeters Billy Butterfield and Pee Wee Erwin, and drummers Cliff Leeman and Barrett Deems, once with Eddie Condon and Louis Armstrong respectively.

Vocalizing will be Maxine Sullivan, noted for her intimate approach, Dave Wilborn whose career dates from the '20s, and local singer Beverly Cosham. The festival continues this afternoon at 12:30 at the Hayloft Dinner Theater, this evening at 9 and tomorrow at 4 at the Elks' Lodge on Center Street.

This year's bash is being taped by National Public Radio for later airing in its Jazz Alive! series.