Deputy Defense Secretary Frank C. Carlucci ordered the Navy and Air Force last week to help drum up business in foreign countries for the FX fighter plane as part of President Reagan's campaign to step up the sale of selected arms abroad.

"We must selectively but actively encourage the foreign procurement of the FX, not leaving this marketing effort just to the manufacturers," Carlucci told heads of the two services.

His memo documents that there has been marked change from the hands-off policy of the Carter administration, which kept the military out of the business of selling the FX. Carlucci said the Navy and Air Force might even give a foreign customer information usually kept secret to help close a deal.

President Carter called in 1980 for manufacturers to develop less sophisticated versions of modern U.S. fighter planes for export with their money and to sell them without direct government help. General Dynamics has built a downgraded version of its F16 fighter-bomber while Northrop last Sunday rolled out its F5G Tigershark to vie for the same market. Neither has been sold abroad, although a deal with Peru is under negotiation.

In giving the administration's rationale for government promotion of FX sales, Carlucci wrote the Navy and Air Force secretaries that many countries are about to modernize their air forces and "it is clearly in U.S. national security interests to have our friends and allies equipped with systems that will still be militarily capable in late 1980s and into the 1990s . . . .

"Together with the Department of State," Carlucci said in the July 29 memorandum, "we have identified the following countries and regions as strong candidates for FX procurement because of a combination of threat, absorbability and fiscal resources: Turkey, Egypt, Jordan, Malaysia, Philippines, Thailand, Indonesia, Bahrain, UAE (United Arab Emirates), Oman and Saudi Arabia (as replacements for F5Es.)

"It is also likely that, as our post-Falklands policy toward Latin America is clarified," Carlucci said, "we shall want to promote the FX selectively in our own hemisphere.

"While recognizing the funding restrictions on the use of Department of Defense appropriations for FX activities," he continued, "the State Department, Secretary Caspar W. Weinberger and I fully expect that whenever possible and appropriate, your departments will encourage representatives of foreign governments and defense establishments to include the FX in their aircraft modernization plans."

Perhaps in response to the coolness the Air Force has exhibited toward buying the FX for itself, Carlucci warned in his memo to both services that "your strong support for the U.S. government position vis-a-vis the sale of FX in dealing with the Congress is expected, regardless of which FX candidate is selected by the country.

"When necessary," Carlucci told the Navy and Air Force secretaries, "you should actively support exceptions to the National Disclosure Policy if such an exception is required either to provide information to a prospective buyer or to complete a sale that has been determined to be in the U.S. national interest."

Noting that the U.S. government has already told foreign governments what the FX aircraft could do, Carlucci said, "We now must go further and actively plan with the nations for sensible acquisitions."

Northrop executives have sought help from top Reagan administration officials to sell its F5G Tigershark, according to several Pentagon officials. One defense executive, declining to speak for attribution, said presidential counselor Edwin Meese III had conveyed to him White House interest in finding buyers for the F5G.

Meese, when asked about this, denied he had asked the Pentagon to buy the Northrop F5G but said through a spokesman that he "admires the airplane."

Thomas V. Jones, chairman and chief executive officer of Northrop, has been a heavy Republican campaign contributor and participant at social functions with President Reagan.

Northrop had high hopes of selling its F5G to Taiwan, but the Reagan administration blocked that sale on grounds that country did not need such an advanced airplane. The company has invested millions in developing the F5G to gamble on the foreign market which so far has proved elusive.