The special presidential envoy to Central America, Richard B. Stone, failed today in his first formal effort to meet with representatives of the Salvadoran left because of last-minute hitches, reliable sources reported.

An official close to the attempt to promote discussions between Stone and the Salvadoran rebel leaders Guillermo Ungo and Reuben Zamorra said that "for the time being" the effort has fallen through, marking at least a temporary setback for Stone's declared mission of promoting contact between the Salvadoran rebel leadership and the U.S.-supported Salvadoran government and its peace commission that was set up to bring the leftists into the election process.

The official did not specify what prevented the meeting despite a day of off-and-on contacts here. Earlier reports had spoken of dissatisfaction by the Salvadoran leftists at U.S. attempts to limit the talks stictly to possible leftist participation in Salvadoran elections tentatively scheduled for December.

"The possibility of a dialogue between the FDR-FMLN and Stone had opened up," said a high-ranking source who requested anonymity. "Apparently, this time, this goal did not crystalize."

FDR are the Spanish initials for the Revolutionary Democratic Front, the political arm of the Farabundo Marti National Liberation Front, the umbrella organization of five guerrilla groups fighting--with support from Nicaragua and Cuba--to overthrow the government in El Salvador.

Costa Rican President Luis Alberto Monge has tried to foster the talks in keeping with his country's self-declared role as a neutral party in the growing conflict of Central America. Authorities here said that as part of the effort Costa Rica arranged the entry yesterday of Ungo, chairman of the FDR's Political-Diplomatic Commission and Zamorra, a member of the seven-member commission, who frequently serves as its spokesman in Washington. "We believe we should shout, but not shoot," said an official. "Therefore, Costa Rica offered its soil as a place to talk."

A spokeswoman for the U.S. Embassy in Costa Rica declined to comment on the reported breakdown, maintaining the strict silence imposed as part of Stone's tactics.

U.S. officials in San Salvador said earlier that Stone planned to return to the Salvadoran capital Sunday. He held talks there Friday, just before coming here, with President Alvaro Magana and members of his government and the Salvadoran Constituent Assembly.

The subject of contacts with the Salvadoran left has been touchy in the tense climate of San Salvador. Rightist political leaders, led by former major Roberto D'Aubuisson of the National Republican Alliance, reacted with outrage last fall when U.S. Ambassador Deane R. Hinton suggested the possibility of "dialogue" with the left as a way to reduce the violence that has killed more than 35,000 persons in three years of civil war.

D'Aubuisson said in a news conference earlier this week, however, that he would welcome contacts by Stone with the exiled left and would accept advice to the Salvadoran government based on these talks as long as they remained within the context of leftist participation in elections. This also has been the Reagan administration's public policy, firmly rejecting any talks on guerrilla power sharing.

The guerrilla forces have insisted that they cannot participate in elections for fear of rightist attacks and because the polling would be conducted by the present government whose legitimacy they contest. Their stand aims at negotiations for leftist participation in the government prior to any elections.