Pick over the raspberries for any leaves or unusable fruit. There should be 3 packed quarts. Combine with the sugar in a large bowl and let stand at room temperature for 6 hours or overnight in the refrigerator. Stir occasionally. Stir together the raspberry mixture, pectin, and lemon juice in a large shallow nonreactive preserving pan and place over high heat.
Bring to a full boil, skimm off foam, then lower heat to medium. Stir often so the bottom does not burn. When the jam is sufficiently thickened, the bubbles on top will become smaller and the jam will look like bubbling tar. This takes approximately 20 minutes, depending on the size of the pan. If the jam is still too liquid, continue cooking for 5 minutes more and test again. Turn off the heat and remove a tablespoon of jam to a small plate to test.
Chill in the freezer for 5 minutes. If the jam is the consistency desired, turn the heat to low while ladling the jam into hot sterilized jars. Wipe rims clean with a damp towel. Seal with new lids and metal rings. Process in a hot-water bath for 5 minutes. Remove, cool, check seals, label, and store.