Rattling my cage!
 

I am helping my son try to adjust the front derailler of his "new" used bike. Gearing:Shimano Ultegra 9, crankset 53/39, rear cassette 27/12. When on the large front to small rear the lateral side of the derailler cage rattles the outer part of the chain. When I adjust the outer derailer screw by undoing it any further the chain falls off laterrally when I try to shift from front small to front large (something I do when not on the small rear cog). I have read about trimming and can't seem to get it to work. Is there some other adjustment I can make to stop this irritating rattling? Is this an intrinsic problem with this set up - this seems counterproductive to me.

Thanks

http://www.parktool.com/repair/byregion.asp?catid=53

The instructions in here should be followed exactly from beginning to end. It could be cable tension, or height of shifter on the seat tube - sorry I cant give a specific answer.