europa

2007 Shimano 105 shifters, specifically the front shifter.

The bike is irrelevant because the entire shifting mechanism was taken from a wrecked bike and placed on its current bike. This was done by a bike mechanic with a good reputation as part the insurance deal (don't ask, it gets complicated).

Sometimes, and I don't know how often because the rider doesn't need to shift chainrings very often, the front shifter jams, NOT the derailleur, the shifter. Typically, she's got the chain onto the large ring but the release won't trigger to drop it back onto the small. If you really force the small lever, you can the mechanism 'click' but nothing happens. Eventually, after forcing both levers to 'click', it'll release and move the dr across so the chain can drop onto the small ring. The shifting then works perfectly until the next time it does it.

The limit screws are adjusted properly. The dr sits a nice distance above the large ring and is parallel with it. The dr clamp is tight and shows no sign of having shifted.

It's possible the cable tension is too tight however, there are NO adjusting screws in the cable system, the only way to adjust the cable is to loosen the clamp and move it. Why? Bloody stupid in my view, but that's how it is (this is where we think back to the mechanic who built the bike up last time). For this reason, I'm unwilling to go messing with the cable clamp unless I'm pretty sure it's the way to go, in which case, I won't hesitate.

Any thoughts?

dsaul

If the shifter is forced to lock into a detent against the tension of the cable, which is against a hard stop on the derailleur, it will require significant pressure on the shift button to get it to release.

Loosen the high limit screw, a bit, and that should relieve the excess tension on the shifter. If that doesn't result in the chain being able to fall off the outside of the chainring, you're set. If it fixes the problem, but the chain can fall off the chainring, you need to reset the high(outer) limit and loosen the cable.

europa

Thanks mate. Pretty much as I suspected. I'm not willing to push the upper limit screw further over but the cable tension is too high for my liking so that's what I'll address. However, I'll see if I can get hold of an inline adjuster before I do, if for no other reason than to make mucking about with it easier.

Iride01 

If it is the high limit screw on the FD, then a cable adjuster won't help. Unless the plan is to always be adjusting it before shifting into and out of the big ring.