The nation's big city mayors had a little trouble yesterday getting their act together.

The topic was President Carter's forthcoming urban aid policy and those parts of his budget that affect cities.

Mayor Coleman Young of Detroit was asked at a breakfast with reporters how he thought Carter was doing on the urban issue.

"I think he's done very, very well," Young replied. "He's focused in on the urban crisis."

Three hours later Mayor Lee Alexander of Syracuse was telling the press, "Mayors all over the country are alarmed over the economic philosophy reflected in President Carter's budget."

In fact, he said, the Carter budget is not much better in meeting urban needs than President Ford's budgets were. When Carter presents his urban initiatives to Congress in March, he should ask for another $11.3 billion to finance them, Alexander said.

Alexander is president of the U.S. Conference of Mayars, an organization that represents 750 cities with at least 30,000 population each. Young, an early Carter supporter in the 1976 campaign, is chairman of the conference's urban economics committee.

Their difference in viewpoint suggests some of the reasons why the administration has had difficulty formulating an urban policy. The federal government already spends $50 billion to $60 billion on urban areas, and the questions are whether that much is well spent, whether more is needed, whether current programs can be made to work better, and whether others are called for.

Big city mayors, as a group, cannot usually command a majority of votes in Congress, and the Alexander Young split also raises the question of how effective mayors will be this year in lobbying for more urban funds than Carter is seeking.

Young, whose city receives massive doses of federal aid, praised the budget for increasing job training, public works, community development grant requests and for shifting federal emphasis from suburbs and new cities to older cities with high unemployment.

Alexander, however, called for a reversal of Carter's policy that federal spending should represent less, rather than more, of the nation's total economic output. The president has proposed cutting the federal share of national output from 22 to 21 percent.

With such policy, Alexander charged, "we can't have national health insurance, welfare reform or full employment. These things demand that the federal government take a larger share of the action."

Asked about Young's comments, Alexander insisted that he was speaking for the mayors generally. He said he had not polled them, but added, "I talked to some mayors" before issuing his statement, which he said reflects the conference's policy adopted last June in Tucson.

In an 89-page study of the budget, the Conference of Mayors said it found Carter's economic philosophy "alarming" because "it does not include a strong role for local government, a national rate of economic growth which is high enough to meet the needs of cities, nor the proper mix of federal resources to support the new social initiatives promised by the president."

Included in its $11.3 billion supplemental budget for cities, the conference listed an extra $4 billion for public service jobs, $1.9 billion for youth employment, $1.3 billion for the Economic Development Administration, $1 billion for public works maintenance and repairs and $1 billion for urban mass transit capital spending and operating subsidies.