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San Francisco 49ers outside linebacker Dan Skuta (51) attempts to tackle St. Louis Rams quarterback Austin Davis (9) during the first half at the Edward Jones Dome. Image Credit: USA TODAY Sports

St Louis, United States: Colin Kaepernick sparked San Francisco with an 80-yard touchdown bomb as the 49ers overcame a slow start to beat the St Louis Rams 31-17 on Monday.

San Francisco trailed 14-3 just seconds before half-time, when Kaepernick rocketed a long strike to Brandon Lloyd for a game-changing score.

“We had Brandon on a double move, we liked his matchup and he did an amazing job running and making the catch,” Kaepernick told reporters of the big play. “We had to make plays.”

The 49ers tallied 24 straight points to take control, then finished off the Rams in the final minute, when Dontae Johnson returned an interception 20 yards for a score as St Louis were trying to put together a tying drive.

It was the third straight win for San Francisco (4-2), who came into the game amid speculation about head coach Jim Harbaugh’s future with the team. The victory put them ahead of Seattle and just behind division leaders Arizona in the NFC West.

Kaepernick took the lead role with 343 yards passing and three touchdown throws. He added 37 yards rushing.

“I think his decision-making [has really improved],” 49ers wide receiver Anquan Boldin said of Kaepernick.

“He has always been a good leader. But he has really grown in decisions, when you see him going through his reads.”

St Louis (1-4) lost their third straight despite coming out strong with an 80-yard drive that ended with Benjamin Cunningham scoring a touchdown from one yard out.

St Louis quarterback Austin Davis then hit Lance Kendricks on a 22-yard touchdown that gave the Rams a 14-0 edge after the first quarter.

Phil Dawson kicked a field goal to get San Francisco on the scoreboard midway through the second, before Kaepernick came alive.

The 49ers’ signal-caller stayed hot in the third, where he connected with Boldin and Michael Crabtree on touchdown passes.

The Rams pulled within 24-17 late in fourth and got the ball back with about a minute remaining, but the interception doomed their chances.

Davis finished with 236 yards passing but completed just 21 of 42 passes. He was sacked five times by the 49ers defence.