The U.S. Census Bureau, acting after a review requested by D.C. City Council member David A. Clarke (D-Ward 1), yesterday adjusted its 1980 census count for the District by adding 1,244 "new" residents to Clarke's central city ward.

The change gives Washington a total population of 638,895. In Clarke's ward, which appeared destined under the old count to be a target for drastic boundary changes, the official population count is now 78,683.

The original census figures indicated that Clarke's ward, which includes the Kalorama and Mount Pleasant areas and extends eastward to Howard University, had recorded a population loss of 17.7 percent over the last decade.

The revised population count for the ward reflects a 16.2 percent decrease over the last 10 years, a change seen by officials as a reflection of the changing character of some Ward 1 neighborhoods as younger singles and childless couples move in and other, older residents with larger families leave.

Clarke made the request for the review last March after a random survey had been conducted in several Ward 1 blocks. The survey indicated that many persons had been missed in the count.

For example, the Campbell Heights housing complex for the elderly on 15th Street NW has 170 occupied units. The original 1980 census figures for that block showed a population count of only 15 persons, it was reported.

The original count also showed zero population for two blocks of Ward 1 that Clarke estimated had a total population of 342 residents.