I just upgraded to 105 10 speed STI shifters on a bike that was originally a 7 speed downtube shifter system with double chainring. Things I changed to 10 speed 105: shifters, free hub (actually changed the whole wheel), cassette, rear derailer, chain. I did not change the front derailer and chainrings. The new 10 speed rear gears work fine but I have a problem with the front gears.
The front STI is double and triple compatible. As I have a double, I should only need to make one shift but this does not complete the shift to the large chainring. It only shifts when I do the second shift. Effectively, it acts as if I have a triple and I have to shift from small to "middle" to large to complete the shift instead of just one shift from small to large.
The problem cannot be caused by the following: I am not confusing a trim click as a shift (there is no trim click on the large lever anyway). There is no slack in the cable when on small ring and is routed correctly over the tab on the derailer. The low limit screw is set correctly and the inside of the cage barely clears the small chainring. When I make the first shift, the cage does move the chain quickly but just not far enough to mount the large ring.
1) What else can it be and how can I fix this?
2) Is this simply because the spacing on a former 7 speed chainring is just too wide for 10 speed STI front shifter so I just have to live with making 2 shifts?
3) If I can't fix it, will it damage the STI shifter to shift twice to make a single shift from small ring to large? The instructions warned against shifting to the third position on a double.