Foltynewicz (7-4) struck out six and allowed five hits and a walk for just his second quality start in eight games.
His only mistake came in the second inning, when Brandon Crawford socked an 0-1 fastball for his 12th homer. Crawford also doubled.
San Francisco fell to 13-26 since the All-Star break, the worst record in the majors over that stretch. The Giants were seeking their first three-game winning streak since winning four in a row just before the break.
Kemp homered in the fourth to make it 3-1 against emergency starter Albert Suarez (3-2). Suarez was called up from Triple-A Sacramento to start in place of Jake Peavy, who went on the 15-day disabled list with a lower back strain.
