Wash plums, cut in pieces, and remove pits.
Crush fruit and measure.
Combine plums and sugar in large pan; bring to a full rolling boil, stirring constantly.
Boil hard for 7 minute.
Remove from heat; add pectin at once.
Skim off foam and alternately skim and stir jam for 5 minutes to prevent fruit from floating.
Ladle into hot scalded glasses and seal at once.