Jewish leaders urged a boycott of a speech tonight by Black Muslim leader Louis Farrakhan after Syracuse University's student government refused to cut off funds for his campus appearance. In an emergency meeting Saturday, the Student Government Assn. at the New York school voted to allow Farrakhan's appearance, saying to do otherwise would be to stifle free speech. Critics of Farrakhan, saying he is anti-Semitic, had asked the association to rescind an estimated $6,000 it supplied to the Student Afro-American Society for a speaker. Meanwhile, Jewish leaders said they plan to mount a protest during the speech but said they would not attempt to block Farrakhan's address.