Chain overshifting on big ring

soveda
soveda Posts: 306
edited May 2010 in Workshop
There is probably a really basic reason for this but on the last two rides I have been on I have managed to overshift so that the chain ends up over the big ring and drops onto the pedal spindle.

It never used to happen and I have now moved the limit screw a bit so that the mech doesn't move quite as far but that means that I have lost the trimming click I used to have.
I'm running Tiagra with a compact chainset and a 12-27 cassette, the only recent changes have been the cassette (was 12-25) and a new chain (now KMC, was Wippermann). Could those be the culprits (could the chain be too long?) or am I just trying to shift too quickly?
Oh yes, the chain is on the 14-15 cog when it happens.

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