Combine plum pulp and fruit pectin in a Dutch oven. Bring to a boil; cook 1 minute. Stir in sugar; return to a boil. Reduce heat; cook 30 minutes, stirring frequently, or until jam thickens and reaches desired consistency. Skim off foam with a metal spoon; discard.
Ladle jam into hot, sterilized jars, leaving 1/4-inch head space; cover at once with metal lids, and screw bands tight. Process in a boiling-water bath 10 minutes.