Leslie Burr was floating on Cloud 9 last night after guiding Edwin C. Andrews' 6-year-old gray gelding, Chase The Clouds, to victory in the $25,000 President's Cup Grand Prix, the final event of the Washington International Horse Show at the Capital Centre.

Burr, 23, of Darien, Conn. was one of 13 world-class riders in an original field of 32 to qualify for a timed jump-off over eight jumps.

"It's beyond words," said Burr. "He (Chase the Clouds) was so good to me. He has been such a phenomenal horse all year long."

Burr was the seventh rider to enter the jump-off and finished in a blistering time of 32.07 with no faults. The only other rider to previously have no faults was Bernie Traurig on Eaden Vale, who was the first to go in the final round, finishing in 33.40.

"It's not always the worst to go first," said Traurig of Hartland, Wis., who also finished as the national rider champion with 21 points. "It can be the best, if you put the pressure on everybody else."

"But I knew that the double-jump 5A and B was dangerous . . . two people almost broke their necks there."

Traurig was referring to two yellow jumps that were each five feet in height and five feet wide. This double effort took its toll bringing down last year's winners, John Simpson on Texas and ace veteran Rodney Jenkins on Second Balcony, who fell with the horse and had to walk out of the arena without remounting.

The only other rider to go clean in the jump-off was Conrad Homfeld on Balbuco who finished in a time of 37.14, good for third place.

Burr is in her first year of international jumping competition. She was the winner of the national equitation finals at Madison Square Garden in 1972. She weighs 127 pounds and had to carry enough lead weight under the saddle to weigh in at the required 165. She said that she had been on a diet this week and was 1 1/2 pounds under the required weight even with the lead. "But the rules allow three pounds for the bridle."