Cubs starter Paul Maholm (0-2) surrendered six runs in four innings. The left-hander is 0-7 with a 7.03 ERA in nine starts since his last win on July 10, 2011 against the Cubs when he was with Pittsburgh.
Carpenter has been getting regular duty while Lance Berkman recovers from a strained left calf and is batting .409 with 10 RBIs in 22 at-bats. He singled his first two trips with an RBI, belted a two-run shot to straightaway center off Lendy Castillo in the fifth for his first career homer and added a two-run triple off Rodrigo Lopez in the seventh.
Berkman missed the weekend series but could be back Tuesday when the Cardinals open a three-game series against the Reds.
Westbrook (2-0) dropped 20 pounds in the offseason after a disappointing 2011 in which he went 12-9 but with a 4.66 ERA and was virtually unused in the Cardinals' World Series championship run. He pounded his sinker for 12 ground ball outs, and allowed four hits.
Reed Johnson had two hits and an RBI for the Cubs, who scored their second run on a wild pitch. They were outscored 15-4 the last two games after hammering Adam Wainwright in St. Louis' home opener for a 9-5 victory.
Westbrook allowed an unearned run in seven innings in his first start at Cincinnati. He's gotten plenty of support, with St. Louis scoring 17 runs his first two outings.
The Cardinals needed one hit to score twice in the second, with Carlos Beltran hit by a pitch and Molina walking setting the table to start the inning. Carpenter followed with an RBI single and Shane Robinson beat out the relay on a double play ball for a run-scoring groundout.
Rafael Furcal had been 0-for-12 before doubling to start a four-run third capped by Molina's homer.
The Cardinals are 12-6 against the Cubs the last two seasons and 8-2 at home.
Notes: The Cubs had been 8-for-10 on steals before Byrd was thrown out. ... Westbrook threw first-pitch strikes to nine of his first 11 batters. ... Two members of the Cubs rotation have no wins since July 10, 2011, with Chris Volstad 0-6 with a 4.17 ERA since beating the Astros. ... Beltran is batting .429 (9-for-21) with two homers and six RBIs against Maholm.
Link to boxscore: St. Louis 10, Chicago 3