When my chain is on the big front ring and big ring in the back (its a double, 12-25 ) if i backpedal when stopped or roll the bike backwards the chain jumps to the small ring on the crankset. I've tried adjusting the barrel on the rear derrailleur no luck, LBS couldn't fix the problem. I know not to use this combo with the chainline issues, but sometimes i'm just in that combo. It was working properly and the only thing I changed was a 50t ring to a 51t for the front then this problem started. LBS said live with it or change back to 50t, I'm sure there is a solution. Any help would be appreciated,thanks.