Metro has delayed until mid-May its scheduled opening of bids on a new order of about 90 subway cars that will be needed when the system expands to Alexandria and Fairfax County late in 1981, General Manager Theodore C. Lutz said.

The month delay, Lutz said, is necessary to permit a change in the specifications on the cars' control equipment. Metro had specified a new type of controller called a chopper, which returns electricity generated during braking to the power supply.

The previous model, called a cam controller, dissipated electricity as heat through a bank of resistors.

Technical problems with choppers on news cars, however, have been discovered by transit authorities in Chicago and Atlanta. Metro is anxious to avoid those problems so it can guarantee that reliable cars will be available when needed.

As a result, the cam controller will be ordered for most of the new cars, while the chopper will be specified for a few new cars that would be delivered late in the order. Metro is expectingas many as four bids on the cats, but only one from an American firm.

In other business at its meeting last week, the staff proposed a test of a different grade of diesel fuel for Metro buses to see if fuel costs could be cut and fuel availability increased.

The proposal was one of several suggestions made after Metro received only one bid-from Gulf-to supply the No. 1 diesel fuel used by Metro buses. Number 2 diesel fuel, which is cheaper, burns less cleanly and is smokier, according to the report. However, some transit authorities have cut fuel costs by mixing No. 1 and No. 2 diesel, the process that Metro will try. Metro also plans to increase its fuel storage capacity.

In other action, the staff promised Metro Board Chairman Jerry A. Moore Jr. a report on the operation of the Metor police radio system. Moore said he had received reports that the radios do not work well in all locations.