Japanese Prime Minister Takeo Fukuda told a press conference yesterday he had promised President Carter "massive efforts" to reduce Japan's $14 billion current account surplus.But Fukuda refused to be pinned down on earlier Japanese forecasts that the surplus would be cut to $6 billion this year.

"I have no doubt there will be a substantial reduction in the volume of exports, "but the amount in dollars depends on inflation in the United States," Fukuda said.

American sources said there has been some slippage in prospects for reduction of the Japanese current account surplus (trade and services), and an optimistic appraisal now would be a surplus of about $9 billion for calendar 1978.

The 73-year-old Japanese leader spent about 3 1/2 hours at the White House with President Carter and key American officials - almost an hour more than originally scheduled.

Conservation groups protesting the killing of whales and porpoises were only barely audible in Lafayette Park as President and Mrs. Carter escorted Fukuda to the White House front portico after a working lunch.

Both sides agreed that the discussions had been especially warm, including reaffirmation of friendships, and reassurance to Asian nations - through Fukuda - of the continuing American interest in the Asian continent. Fukuda said that Carter's reassurances "will be conducive to [WORD ILLEGIBLE] Asia."

[WORD ILLEGIBLE] pledges and [WORD ILLEGIBLE] his government made [WORD ILLEGIBLE] through an agreement between Special Trade Representative [WORD ILLEGIBLE] Strauss and special ambassador [WORD ILLEGIBLE] Ushiba to open up Japanese markets in an effort to slash the excessively heavy Japanese surplus with the U.S. in particular and the world in general.

After 2 days of discussion, American officials came away totally convinced of Fukuda's commitment to do his utmost to accelerate Japanese growth, and to accede to American wishes to avoid an inundation of the market here by Japanese goods.

Fukuda said that steel exports would be cut by 10 percent to 20 percent, color TV shipments by 30 percent, and "although American citizens, we know, want to buy Japanese cars," shipments will be held below last year's "through administrative guidance."

He acknowledged that many parties, including highly regarded statistical agencies in Japan, doubt that his country can hit a 7 percent growth rate this year. "But I disagree," he said with a smile, "because I have mobilized every possible measure to reach (that figure.)"

One new element that came out of the sessions was an agreement to strengthen consultations on developments in foreign exchange markets. Although described by White House spokesmen as "similar" to an arrangement worked out with West Germany, officials hastened to emphasize that there was no change in U.S. intervention policy, which is to intervene only to correct "disorderly market conditions."

So far as is known, only Japan has intervened to prevent the dollar from dropping against the yen.

Both the White House and Fukuda also announced a series of short-term measures designed to provide some quick reductions in the Japanese trade balance until longer-term, and more permanent, changes can be made effective.

The short-term measures include a Japanese proposal for prepayment of uranium enrichment services (Fukuda said Carter's reaction was "very favorable"); increased purchases of commercial aircraft (DC-9-80s); increased buying of non-ferrous metals; and stockpiling of oil.

President Carter told reporters that the meeting "went fine." He was said to have pressed Fukuda especially to improve trade concessions offered in Geneva in so-called Tokyo Round of multilateral negotiations.

He also stressed that the United States is expecting Japan to make good on that part of the Strauss-Ushiba agreement (Paragraph 6) which calls on Japan to give U.S. exporters the same access to their markets as the U.S. gives to Japanese exporters.

At his press conference, broadcast via satellite to Japan, Fukuda said his friendly conversation with President Carter had covered worldwide topics, and that the time frame had even extended into the 21st Century.

He said they had discussed ways of avoiding a continuance of the chaotic conditions that had stemmed from the "oil shock" of 1973, and had agreed that the upcoming economic summit session in Bonn "must" be a success. The Japanese are anxious to get firm antiprotectionist language into the summit communique.

Pointedly. Fukuda referred to the pledges all nations had made - but not fulfilled - at the London summit last May, including Japan's own promise of a high growth rate and a reduction in its surplus - which never eventuated.

But he withheld any significant promises on trade concessions in Geneva, saying each country asks for more, and that the negotiations are still going on.