Those bicyclists who want to raise $50,000 to aid victims of the 1985 Mexico City earthquake are learning that it is harder to give than to receive.

Save the Children, the international charity that has been sponsoring their efforts in this country, has grown wary of them and their ambitious plans. It has ordered the volunteers to vacate by tomorrow the office space on Connecticut Avenue that was lent to them.

"Save the Children don't want us to use their name," said Adam Baines, the London ad executive who started Operation Mexico, the Maine-to-Mexico benefit bike ride, after raising about $40,000 in England with a concert by Placido Domingo. "They're afraid we'll try something too big and won't be able to bring it off, and they want us to move to another charity. But it's too late for that."

Baines and his two companions have been in Washington for the past week, using the office facilities of Save the Children and lodging with friends. They plan to stay another six weeks while they launch two benefit concerts, one with the local opera community and the other "a contemporary music event."

"I have a letter from Save the Children headquarters," Baines said. "They want to have close supervision and coordination of such a big thing. For instance, what if we brought in a controversial band or something? They want to keep close tabs on us."

At the Westport, Conn., headquarters of Save the Children, Director Robert Burch said that his organization is proud of its high ratio of funds actually delivered to the needy.

"To date, Baines and his group have sent us $475 from their work in this country, and that's fine, but we have to be very careful. He's sent us a summary of his projects, but we need to check them out. We don't want to curtail any sincere effort, of course. He asked us if he could continue to sell the T-shirts with our name on them, and we said to go ahead."

After a call from Save the Children's headquarters in England several months ago, Burch had agreed to support Operation Mexico with the understanding that the bikers would be working with the opera communities in various American cities.

"We don't have that kind of manpower," Burch added, "though we said we'd give them a letter of support."

But when Baines and his companions reached Washington and contacted the Mexican Embassy, they learned that the Mexican government was not interested in receiving charity, that "the damage is 98 percent rebuilt," as an embassy spokesman said. Since then Operation Mexico has been trying to regroup.

According to Alan O'Donnell, a folk-singing cyclist from Ireland, Operation Mexico expects to raise $20,000 from events planned in Washington. He said these could be announced by tomorrow. Meanwhile, he said, there is almost $1,000 in checks on hand and another thousand coming in from the T-shirts, but this money has not yet been turned over to Save the Children.

"The situation is unclear," he said. "The checks we have already turned in haven't cleared yet, and we want to find out about that first. Save the Children takes 20 percent for administration and sends the rest to Mexico. We do not get paid at all. We're volunteers giving our own time and money for this."

The funds go to a housing project in Mexico City that requires another $100,000 for completion.

"It's very difficult to find another charity to work with at this point," said Baines. "But we're going ahead. We're not stopping.