It sounds like you need to tighten the cable. It might be difficult if you don't have a barel adjuster. You might want to check and see if there is an adjuster on the shifter end of the cable. If not, you need to loosen the bolt that attaches the cable to the front derailure and take in a little bit of slack on the cable. You'll want to loosen the cable as much as possible before dint this. Your cable might be really freyed where it attaches to the derailure. If this is the case, you can buy a new one. Wall mart sells them.