Follow steps 1 through 4 of Canning Instructions, using seven pint-size jars.
Coarsely chop plums (do not peel); you should have 7 3/4 cups. Coarsely chop or mash raspberries; you should have 2 cups.
In an 8- to 10-quart pan, combine plums, raspberries, lemon juice, butter (if using), and pectin. Bring to a rolling boil over high heat, stirring often. Stir in sugar; when mixture resumes boiling, stir for exactly 4 minutes if using MCP pectin (1 minute if using Sure-Jell pectin).
Skim and discard any foam from jam.
Follow steps 5 through 11 of Canning Instructions, leaving 1/4 inch of headspace in each jar and process jars for 5 minutes (see notes).
Sunset's Canning Tips:
Add butter to jams and jellies to prevent foam from forming during cooking. If you omit the butter, skim off the foam before ladling jam or jelly into jars. The recipe will yield about 1/4 cup less.
Measure all the sugar into a bowl before beginning the recipe. Many canning recipes call for a large volume of sugar to be added when a mixture is already boiling; measuring ahead simplifies this step and prevents mistakes.
Use a ruler to measure volume. Some recipes call for a mixture to be reduced by a certain amount. To ascertain this easily, insert a clean, wood ruler into the pan before cooking and measure how far up the mixture comes. Then cook as directed until it has reduced by the percentage specified. For example, if uncooked mixture measures 4 inches in pan and recipe says to reduce by half, cook it down to 2 inches.