In a food processor or blender, purée peaches to make 4 cups (1 L). In a non-reactive saucepan, stir together peach purée and lemon juice. Stirring continuously, slowly add pectin. Let stand 20 min., stirring every 5 min. to blend. Stir in corn syrup, then stir in sugar until well mixed.
Cook peach mixture over low heat until just warm to the touch, about 100ºF (38ºC); do not allow heat to increase further. Pour into tight-sealing food containers to within 1/2 in. (1 cm) of top. Cover and let stand at room temperature until set. Store jam in freezer. To use, thaw in refrigerator. Once thawed, stir jam to return to smooth consistency; keep refrigerated.
To peel peaches easily, mark a shallow X on the bottom of each one. Submerge peaches in boiling water for 3 to 4 min. Transfer to cold water, drain and slip off peels.