Will mountain bike front derailleur work with Shimano Tourney index shifter?
In case anyone is interested, I got the following suggestion on www.bikeforums.net. The limit screws are not an issue, but I bet flex is. I'm going to check it out.

"Since the front shifter works on the tensioned part of the chain, it does take more force for front shifting. If you have the range on the stand, but isn't getting all gears on the road, then either something is flexing, or the limit screws needs to be backed out some to allow a little more overshoot."

Here is a link to that thread: 
http://www.bikeforums.net/bicycle-mechanics/1118824-shimano-tourney-shifter-work-mountain-bike-derailleur.html#post19798217

Here is an update: 

I ended up swapping out the front derailleur, which made it possible for me to shift to all three front rings. However, it was laggy and unpredictable. That got me to look closer at the bottom bracket. I have a bit of slop there, which is likely the final thing -- I hope -- needed to fix this. Alas, I am now thousands of miles from my bike and won't be reunited with it until mid-October, so trying that will have to wait.

Another up date: my front derailleur now shifts great. I repacked the bottom bracket and eliminated that bit of slop. However, I think the biggest factor was that remounting the front chain ring and crank arm actually moved the front gears closer to my frame. I can tell it ended up closer because my limit screw setting was off. Not by much, mind you, but I didn’t need much. This is an old bike; the pedals are a press fit onto the bottom bracket shaft, essentially a square wedge going into a square, tapered hole,

Anyway, now I’m having trouble with my rear derailleur, but that’s a problem already on the way to being solved.