The Carter administration will ask Congress to make any rebates established for purchasers of small cars "retroactive to the end of April." White House energy adviser James R. Schlesinger said yesterday.

Appearing in a question-and-answer session with a group of donors to the Democratic Party.Schlesinger also said the President's energy legislation will set the effective date for additional taxes on gas-guzzling big cars for some time after the measure passes Congress.

Last Sunday, American Motors President Roy D. Chapin, complained that the President's proposed rebate for buyers of small cars was making people delay buying such cars until the rebate became effective.

American Motors is a big maker of small cars.

Schlesinger said the retroactivity of the rebate would ease the problem of delayed buying. He added that the administration did not believe it could legally make the higher tax for big autos retroactive.

During his 40-minute appearance, Schlesinger fielded several questions from oil and gas producers who pressed for reasons why the Carter energy program had not contained incentives for new oil drilling.

"The $13.50 a barrel price for oil, Schlesinger said, "is a generous incentive . . . We urge you to go out and find it."