Front derailleur springs off big chainring

[CanadErik]

I'm tuning my girlfriend's bike, and I've encountered a new problem with her front derailleur. I can use her thumb shifters to pop the chain up onto the big ring, but as soon as I take my finger off the shifter, the derailleur springs back and drags the chain back down to the small ring. The only way to stay in the big ring is to keep the shifter held in (which obviously is a problem). Any help?

[cyclerUK]

If this is a friction shifter then there should be some way of tightening it up.
Usually there is an adjuster on the top (either a screw of "butterfly").
Just tighten that until the shifter holds it's position.

[Joe_W]

Thumb shifters? Probably gummed up. Take a look at http://forums.bicycletutor.com/thread-900.html