Former West German chancellor Helmut Schmidt says that he is not yet convinced that the Reagan administration is negotiating seriously at the Geneva talks on medium-range missiles and that the administration will have to produce evidence of good faith before he can endorse deployment of modern nuclear missiles in Europe later this year.

Schmidt also contends it will be "very difficult" to place powerful Pershing II rockets in West Germany beginning this December if the United States does not resolve the controversy about how and where to base the intercontinental MX missile.

In a two-hour interview with Washington Post Co. Chairman Katharine Graham, Schmidt criticized what he described as the vacillations of U.S. foreign policy during his 8 1/2 years as chancellor and said there has "never been greater neglect for European participation in the alliance than under presidents Carter and Reagan."

Looking relaxed as he sniffed snuff, wearing a polka-dot shirt and no tie, Schmidt said he has never enjoyed life more than since leaving power eight months ago. "I'm getting much more sleep, and I'm now the master of my own timetable," he said.

Nonetheless, he showed that retirement has not mellowed his acerbic candor or the strong opinions he holds about the need for effective leadership in the West.

"As chancellor I worked under four U.S. presidents, and it's quite an experience, I can tell you," Schmidt said. "I've become greatly troubled by your handling of allies and friends."

During his tenure in office, Schmidt's irascibility was known to have worsened some trans-Atlantic quarrels and created exasperation within the four U.S. administrations involved.

"Take the pipeline embargo. There was not the slightest consultation; we learned about it from the evening news," he said. "The grain embargo, the Olympics the 1980 boycott --all these actions show enormous neglect for the alliance.

"First [Jimmy] Carter sent his vice president to tell us almost everything done by his predecessors was wrong and implied that our cooperation was in vain and something different had to start. Then, later, along comes Reagan and he tells us the same thing."

Schmidt chastized the foreign policy reversals carried out by the Carter and Reagan administrations. "The alliance needs continuity," he said. "We've put all our eggs in your basket. We even have learned to keep silent about German reunification, although it's not out of our minds."

Schmidt complained that lack of consultation has also marred allied relations about how to respond to the Soviet buildup of 351 SS20 missiles. The goal of that policy, he says, "is to divide West Germany, in some future crisis, from the United States."

The former chancellor is considered one of the key architects of the North Atlantic Treaty Organization's so-called two-track strategy, which the allies approved in December 1979. The plan called for negotiations to limit medium-range nuclear weapons in Europe and for deployment of 572 cruise and Pershing missiles in case the arms talks fail to reach a compromise.

Last summer, chief U.S. negotiator Paul Nitze and his Soviet counterpart, Yuli Kvitsinsky, reached a tentative deal during a walk in the woods near Geneva to limit missiles to 75 cruise missiles for the west and 75 SS20s for the Soviets. That compromise was subsequently rejected in Moscow and Washington.

"I was never consulted, nor were others, on the repudiation of that plan," Schmidt said. "As I interpret the interests of my country and the West as a whole, the walk in the woods deal was totally acceptable."

Schmidt now fears that the opportunity to strike a deal with the Soviets may have been lost. "It's not too late to reach an accord, but that does not say anything about the probability," he said.

Schmidt indicated he will voice his opinion on the missile controversy once the fate of the arms talks becomes clear. The determining factor for him to support deployment will be proof of good faith by the United States in the Geneva negotiations.

"I am not, as yet, convinced that the Americans are negotiating seriously, but they still have time to do so," he said.

Beyond the immediate missile crisis, Schmidt sees a growing chasm between the United States and Western Europe over how to deal with the Soviet Union as a neighbor and adversary.

"We Europeans want to cooperate with the Soviets, especially on arms reduction," he explained. "Detente seems like a dirty word in some American circles, but I must remind you that it is official NATO language.

"You will not get any European government to repudiate detente," he said. "We want to live in peace without knuckling under to the Soviets.

"My own town of Hamburg has traded with Novgorod for at least 600 years," he explained. "If we traded only with democracies we would ruin our economies very quickly."

He also rebuked the notion that the West can weaken the Soviets with a commercial boycott. "It's a romantic idea to believe you can bring the Soviets to their knees by economic measures. Russians have an enormous capacity, one might even say passion, to suffer for their country, and they will certainly do so to block attempts by foreigners to prevail over them."

The best way to combat Soviet influence, he said, is by utilizing the greater prosperity in the West to promote the economies of developing countries. "Who threw the Russians out of Egypt?" Schmidt said, adding that revolution and Russian influence are thwarted not by weapons, but by economic aid.

"The security threat to the United States in Latin America does not come from Nicaragua but the shaky economy of Mexico," he said. "If you do not put Mexico on a sound economic basis, the United States will soon be flooded by Mexicans pouring across the border."

Schmidt feels a stable world requires dialogue between the superpowers.

"It's a great mistake that Reagan did not see Leonid Brezhnev, and I deplored the fact he did not go to Brezhnev's funeral in order to show all the world he was willing to see Andropov."

Schmidt, who will turn 65 this year, plans to withdraw from the Social Democratic Party leadership but will stay on as a member of parliament from his Hamburg district. He also intends to write several articles as copublisher of the political weekly Die Zeit and perhaps one or two books, but "no autobiographical stuff."