ST. LOUIS -- Carlos Zambrano decided to try a different approach after he struggled during his pregame bullpen session Wednesday night.

Since he was pitching in St. Louis, it worked out just fine.

Zambrano tossed six innings of two-run ball, Tyler Colvin hit a three-run homer and the Chicago Cubs beat Chris Carpenter and the sliding Cardinals 7-3 to complete their first three-game sweep in St. Louis in more than 20 years.

Zambrano (9-6) allowed an earned run and four hits, improving to 6-0 with a 1.37 ERA in his last seven starts. He is 7-1 with a 1.29 ERA in 12 career starts in St. Louis, including 6-0 in seven outings at the new Busch Stadium.

"When I come here, for some reason I feel good," Zambrano said. "I said before the game, any time you face [Adam] Wainwright or Carpenter, you got to come with your 'A' game. Those guys are not easy matchups. They're two of the best in the league, and you got to come dealing, too."

Cardinals starting pitcher Chris Carpenter adjusts his cap while working against the Cubs during the first inning of Wednesday's game in St. Louis. (JEFF ROBERSON ~ Associated Press)

Zambrano was struggling to get his fastball up to 90 mph before the game, so he decided to improvise. He told pitching coach Larry Rothschild he planned to use his breaking stuff more.

"When you see the gun and your fastball is only 88 or 89 you have to pitch with your other pitches," Zambrano said.

It was the Cubs' first three-game series sweep in St. Louis since June 10 to 12, 1988. Chicago also has won five consecutive games at the Cardinals for the first time since June 18 to Aug. 14, 1983.

"I'm thrilled to win games," Cubs interim manager Mike Quade said. "To come in here and play like this against a club that's fighting like a son of a gun to get the division makes it even better."

Colvin connected against Carpenter (15-7) in the Cubs' four-run seventh inning, becoming the fourth Chicago rookie to hit 20 homers in a season. Teammate Geovany Soto was the last to accomplish the feat, hitting 23 in 2008.

St. Louis Cardinals' Skip Schumaker, right, is safe as he slides across home ahead of the tag from Chicago Cubs catcher Koyie Hill on a sacrifice fly by Colby Rasmus during the third inning of a baseball game Wednesday, Sept. 15, 2010, in St. Louis. (AP Photo/Jeff Roberson)

Micah Hoffpauir also doubled in a run in the seventh and Aramis Ramirez added a two-run drive in the eighth.

Carpenter pitched 6 1/3 innings before leaving with a right hamstring cramp. He was charged with five runs and seven hits.

"I felt like I threw the ball well," Carpenter said. "Colvin hit a good pitch. That's what was kind of weird. I was fine the whole game."

Not even the return of Albert Pujols could help the Cardinals, who fell eight games behind Cincinnati in the NL Central with 18 games left on their schedule. Pujols missed Tuesday's game with inflammation in his left elbow, but he started Wednesday and went 0 for 2 with two walks and was hit by a pitch.

"We're struggling," St. Louis manager Tony La Russa said.