While curious Australians hunt for bits of the fallen Skylab and technicians fight repeated delays in preparing America's orbital shuttle for launch, two Soviet cosmonauts tonight set a new record for space endurance.

At 10:42 p.m. Washington time, Vladimir Lyakhov and Valery Ryumin, aboard the Soviet space station Salyut 6, broke the previous mark of 139 days, 14 hours and 48 minutes in space.

Despite troubles that have deepened the long isolation of their flight, pilot Lyakhov, 37, and engineer Ryumin, 39, felt "well" as they approached the record, the Tass news agency reported yesterday from mission control outside Moscow.

Previous Soviet practice of shuttling visitors to the Salyut to relieve boredom and provide human contact for long-endurance crews was thwarted in April when a spacecraft bearing a Soviet pilot and a Bulgarian engineer suffered a rocket malfunction during its final docking approach and was forced to return abruptly to earth. A second Soyuz craft, scheduled to carry a Soviet and a Hungarian for a similar one-week visit with the Salvut crew, was sent up empty instead as a safety precaution while testing new equipment.

But the crew has been visited by several unmanned Progress craft, carrying cargo of rocket fuel, books and newspapers, fresh fruit and vegetables, and a television set so the two can communicate with their families and see them.

Officials reported with detectable pride today that the fresh food and "space meals almost similar to those on earth" have allowed Ryumin to achieve another space first: he is the first man to gain weight in orbit, Tass said, a svelte one pound, eight ounces.

Lyakhov and Ryumin were launched Feb. 25 toward the 19-ton space station, which has been in orbit since September 1977. They are the third successive record-breaking crew to live aboard the relatively cramped orbital station, which has lasted far longer than originally planned.

Because of its relative age and heavy use, some of the Salyut's equipment has been replaced by Lyakhov and Ryumin with new apparatus ferried to them by the Progress cargo missions.

During their weeks aboard, Lyakhov and Ryumin have continued experiments in weightless metallurgy, photography, astronomical observations and scientific measurements of cosmic phenomena. Tass said the crew has carried out "extensive space biology experiments, and even has supplemented its diet of packaged food with fresh lettuce grown in the station."

The Soviets have gone well past the American space endurance record of 84 days set aboard the Skylab five years ago. Two earlier Soviet crews spent 96 and 139 days in space, and Lyakhov and Ryumin apparently will add another few weeks to that latest record. When the time comes for their return, they will use a Soyuz 34 craft that is docked to one end of the Salyut.

The Soviets, who are secretive about much of their manned space program, have not said whether they intend to use the Salyut station after Lyakhov and Ryumin leave. Earlier Salyut models have burned up on reentry and eventually, so will the record-setting Salyut.