Very stiff front derailleur My girlfriend has been having problems with her front shifter/derailleur since she got her bike. For some reason, moving the chain from the middle ring to the big ring at the front, is very stiff.

Her left shifter is an ST-EF50-7 (triple) like the one in this picture  and the front derailleur is a Shimano Tourney TX51 (top pull) 

This is the cable routing to the front derailleur.



Her bike has been used just a few times, and I have inspected the cables and moving parts and everything seems to be ok. No rust, cables have been lubed just in case, etc. Front derailleur is properly adjusted (both low and high screws are set up properly). As long as you push the lever really hard, then everything shifts properly. Something that I have noticed is that in order to shift into a bigger gear, the shift lever has to be pushed quite a bit before it "clicks" and stays in the next gear.

When the cable to the derailleur is not attached to anything, the shifter lever moves smoothly, so I believe that the shifter is fine. It appears that just for some reason, the front derailleur has a very stiff spring that puts up too much resistance. Can anyone recommend me what to do? I'm believing that changing the front mech may be necessary in order to get it working properly.
First of all, if the bike is new and there is a problem with it, just take it back to the store where it was bought and ask them to fix it. Most bike stores in the US offer some sort of tune up after a few weeks to deal with any problems that crop up, re-adjust as cables stretch etc.

If you need to diagnose and fix it yourself:

You have ruled out the shifter, so the resistance must be in the cable or the derailleur itself. I doubt it's the derailleur spring being too stiff. The derailleur lever mechanism and springs are designed to have approximately constant  return force across the range of movement. The inexpensive Tourney line is has some stamped sheet metal parts, there may be a burr or defect that is preventing the derailleur from moving freely.

To check the cable, disconnect from the derailleur at the pinch bolt. While holding it taught at the end work the shifter and make sure the cable moves freely and is not hanging up at all.

With the cable still disconnected, you can manually push the derailleur cage from the small ring to large ring position to see if there is any resistance. It helps of you can pop the chain out of the cage, there should be a small screw that you can undo to open the cage and slide the chain out.