Thirty-eight House members yesterday sent a letter to President Carter asking him to clear up the confusion that they believe is undercutting his decision to cancel the B-1 bomber.

Their appeal went to the White House shortly before the House was scheduled to vote on an amendment to reverse Carter's B-1 decision by appropriating $1.4 billion to put the bomber into production.

But the confrontation on the B-1 was avoided at the last minute by a joint decision by House Speaker Thomas P. (Tip) O'Neill Jr., (D-Mass.) and Chairman George H. Mahon (D-Tex.) of the House Appropriations Committee to postpone the voting until next week.

Mahon said in an interview that he and O'Neill agreed to postpone after reading yesterday's Washington Post story about several challenges to be made to Carter programs when the fiscal 1978 supplemental appropriations bill was called up on the House floor.

"The Speaker called me after reading the morning paper and said, 'It doesn't look like you'll have enough time for that bill'", Mahon explained. "I said, 'You're right'".

Rep. Robert L. F. Sikes (D-Fla.), who had planned to champion an amendment yesterday to restore B-1 production money, said the postponement "has to be an indication that anti-B-1 forces don't have the votes."

Sikes, ranking Democrat on the Appropriations Defense Subcommittee, said the postponement will give administration forces more time to mobilize. "They've got a lot of horses going for them," he said.

Rep. M. Robert Carr (D-Mich.), who drafted the letter to Carter signed by 38 representatives favoring cancellation of the B-1, said, "The confusion can't get any worse than it is now."

The main source of the confusion, Carr said, is the administration's willingness to consider developing a strategic bomber version of the F111 warplane after indicating to Congress that the age of the manned, low-level penetrating bomber was over.

When Carter announced June 30 that he was canceling the B-1 and relying on cruise missiles instead, he "didn't just get rid of an airplane, he got rid of a concept: low-level, strategic manned penetration" of the Soviet Union, Carr said.

However, he said, the administration looked as though it was having second thoughts about that concept when Defense Secretary Harold Brown on Sept. 19 wrote Congress that it would be "appropriate" for the Senate to give the Pentagon $20 million to study a strategic bomber version of the F-111.

"That was a bad mistake politically," Carr said. "Brown's a fine scientist, and could be our best Secretary of Defense, but his political antenna needs some find tuning."

The B-1 confusion underscores that the administration desperately needs somebody who understands the House, Carr said. He said only a former member could really do the job. Brown, Carr added, should hired "a Capitol Hill political heavyweight" to keep Carter's defense policy from unraveling.

"But what's Brown doing" up here about the B-1?" asked Carr sarcastically. "He's flying off to Italy tonight."

The 38 lawmakers began their letter to Carter this way: "As members of Congress wh actively supported your decision to cancel the B-1 bomber, we hope you will be able to clarify the reasoning behind the administration's acceptance of an Air Force proposal to spend $20 million on development of the FB-111H penetrating bomber."

The letter than states that Brown has said analyses showed that the F-111 bomber would be less effective than the B-1, but the cruise missile-B-52 combination would be better than either.