Purée plums in a blender or food processor until smooth. 2 pounds pitted plums should yield 1 quart of purée. Add sugar and corn syrup and blend for 30 seconds. Add salt, blend to incorporate, and adjust to taste. If plums are very sweet, add lemon juice to taste, starting with 1/4 teaspoon. Strain purée and transfer to refrigerator; chill for 2 to 3 hours until very cold. Churn sorbet in ice cream maker according to manufacturer's instructions. Transfer sorbet to freezer to firm up for 2 to 3 hours before serving.
