MR. CARTER SOUNDS like the dieter who vows to start his new regimen right after dinner. He said at his last news conference that he intends to put his brave new policy of restraint in arms exports into effect . . . next year. His squirming demonstrates that the President is well aware of the gap between his earlier readiness to treat the level of arms sales as a measure of national virtue, and his current tendency to use sales in the same way past administrations used them: as an instrument of diplomacy. Nevertheless, he insists, the basic policy is still on track.

Part of Mr. Carter's current unease arises from his post-inagural decision to hold up certain arms transfers, even though the history and momentum behind them made them virtually uncancelable, while conducting his own policy review. This decision ensured that, once the review had completed, the deals that had been held up would surge simultaneously into the public stream of congressional review. This is what is happening now.

In this category lies the prospective sale of airborn radar planes (AWACS) to Iran, a turkey of a deal that Mr. Carter has now suspended, but only temporarily, to satisfy congressional demands for more leisurely consultation. In this regard, we note with dismay reports that the objections of Sens. Robert Byrd and Hubert Humphrey to the AWACS transaction seem merely procedural, not substantive. Also in this new category lie the transport and reconnaissance planes newly contemplated for Egypt. When the Egyptians, after breaking loose from Moscow, crossed the arms barrier to Washington last year, it was understood they would be back for more. The flow of arms to Israel, to be sure, has never really stopped.

Coincidentally, the administration is preparing to open arms-supply relationships with Sudan, Somalia and Chad. Here there is no history or momentum. There is, rather, a large target of political opportunity, the sudden chance to blunt the Soviet Union's quite startling grasp for power across a broad swath of Africa through one erstwhile client, Somalia, and a second, continuing client, Libya.

It must be granted that the administration is playing fair, giving Congress and the public a chance to weigh in before these arms commitments are made. It is right, too, to solicit the cooperation of European allies both in providing weapons and in coping with regional politicals tensions. But there are familiar and serious problems. Supplies sent in an anti-Soviet context to Somalia or Sudan can stir the further Soviet arming of their common rival, Ethiopia. Sales to Somalia could embolden it and enable it to grab the large chunks of Ethiopia and Kenya that it covets. Arms sent to Arab Sudan bear on the Arab-Israeli balance. There's reason to go slow.

It is a plus that the Carter administration has tried to make arms transfers a controlled tool of foreign policy as executed in the State Department. But the diplomats' influence is not always on the side of restraint: There is constant pressure to use sales to buy influence, as now in Africa, and to pay and propitiate the Mideast oil producers (including Iran) on which the United States increasingly relies. The Pentagon retains control of the technical estimates of military threat on which some part of arms-export decisions is based. And, as Northrup's Thomas Jones puts it, official arms-export decisions can be influenced by "the desire to put work in one factory or another or in one state or another, attempts to artificially shore up [arms] companies that have not succeeded on previous programs, or the practice of using foreign sales to spread excessive cost overruns on weapons systems beingdeveloped or produced for American forces."

Mr. Carter, good luck.