In a Dutch oven, combine sugars, plums, water and lemon juice. Bring to a full rolling boil, stirring constantly. Stir in pectin. Continue to boil 1 minute, stirring constantly.
Remove from heat; skim off foam. Ladle hot mixture into seven hot half-pint jars, leaving 1/4-in. headspace. Remove air bubbles and adjust headspace, if necessary, by adding hot mixture. Wipe rims. Center lids on jars; screw on bands until fingertip tight.
Place jars into canner with simmering water, ensuring that they are completely covered with water. Bring to a boil; process for 10 minutes. Remove jars and cool.