Former Sen. John Tower, George Bush's choice as defense secretary, should let recent reforms to the Pentagon's weapon-buying system take hold instead of promising more sweeping changes, says outgoing Defense Secretary Frank C. Carlucci.

Tower also should fight as hard as he can for a 2 percent increase in the Pentagon's budget - and not talk about accepting less - because anything less jeopardizes the nation's security and the future of the all-volunteer force, Carlucci said."The last thing the Defense Department needs is somebody to come in and yank the procurement system out by the roots again and say we're going to reform it or change it," Carlucci said.

"The best reform for the procurement system is stability, predictability. The blueprint is laid out, we've made a large number of changes in the procurement system. Real changes are taking place in the procurement system and I am proud of them."

Carlucci was interviewed last week as he awaited Bush's announcement of the man who would replace him. The 58-year-old defense chief, who said he was looking forward to finding a job in the private sector but had yet to do so, talked at length about what he considered his achievements, as well as the problems he foresaw for Tower. Of the former, he claimed a measure of credit for:

-Guiding the process and setting priorities in late 1987 in slashing the Pentagon budget plans to comply with a White House deficit compromise with Congress.

"We took it (cuts) in places the services didn't like and lost a secretary of the Navy in the process," Carlucci said, referring to the resignation of James Webb over a decision to retire 16 warships.

-Protecting reflagged Kuwaiti oil tankers in the Persian Gulf. Carlucci noted he had just returned from a visit to the Middle East "and our standing could not be higher."

"The fact is that we got a high degree of cooperation from the moderate Arab states. The Persian Gulf (work) has been a major achievement."

-Taking a calm, measured approach to the disclosure of a wide-ranging fraud investigation among Pentagon contractors and private consultants. Carlucci said he had instituted a "total quality management program" to impress on contractors the need for quality products as well as ethical conduct.

"We're looking at more innovative ways to base competition on quality and track record as opposed to pure price," he said.

The defense secretary also cited success in establishing a good rapport with his Soviet counterparts.

On the problem side, Carlucci said Tower would have to fight for a larger Pentagon budget; help develop arms control strategy, and strengthen the North Atlantic Treaty Organization while helping European allies resist "the seductive appeal of Soviet arms control proposals."

If Bush and Tower can't convince Congress to approve a 2 percent real growth increase in the fiscal 1990 budget, the only way to make further cuts "sensibly" would be to reduce the size of the armed forces, Carlucci said.

"It's not a question of we can't afford it. If you look at the domestic programs, they continue to go up at a rate of about 8 percent. Therefore, using that yardstick, I see nothing wrong with 2 percent in defense."

Carlucci, who replaced Caspar W. Weinberger as defense secretary for the last 14 months of the Reagan administration, said he had enjoyed the position but found it truly was "a full-time job."

"On a per-hour basis, I make about as much as my babysitter," he joked, adding the secretary's salary of $99,500 is "lousy."