Originally Posted by
BarracksSi
Almost definitely the upper limit screw. Do like flatballer said.
However, I've had a couple cranks where setting the high limit so close that you could barely fit a hair between the FD's outer plate and the chain didn't prevent the chain from getting thrown. The cranks themselves would start to come apart (the spider was press-fitted onto the spindle and would work its way loose after a few hundred miles) and the ring assembly would wobble as if there was severe frame flex in the BB area (one bike was a CAAD8... flex?? Bah!
).
Shift into a tall gear on the big ring, stand up, and the chain gets tossed when my right foot went down. I let one bike get bad enough that I could do it at will.
Switched to another brand of crank and it never happened again.
What's on your bike?
Campy Chorus 10s, with carbon cranks, circa 2005.
I did a visual check now and the FD definitely clears the big ring by quite a bit. It might have been caused when I switched from standard to compact, and didn't readjust after moving the FD down.