I'm trying to install some Acera trigger shifters to use with my Ultegra derailleurs. The rear derailluer/shifter combo (8 speed) is working great. The front however (3 chain rings) seems to have a problem. I have to push extremely hard on the shifter to move from the lowest chain ring to the second, and it is almost impossible to apply enough force to get from the 2nd to the 3rd chain ring. I actually broke a cable trying to go from the 2nd chain ring to 3rd. I've tried messing with the barrel adjusters at the down tube and at the shifters to change the cable tension but it doesn't seem to have much effect. It almost seems like there is not enough mechanical advantage. Is it possible that the combination of the Acera trigger shifters and the Ultegra front derailleur is not a good one?
I just figured it out... the front derailleur cable wasn't going over the leverage tab on the derailleur correctly. I had it going under and that's why there was no leverage going into the largest chain ring. Here is an image of what I mean by leverage tab:
They're not compatible. Shimano road and mountain front shiters and derailers have different pull ratios. They can be made to kinda-sorta work but will always perform badly, tending to have issues with other chain rub or chain drop depending on what compromise you make. It does sound like you are having other problems with your cables that explain the extreme amount of force required to move them.
I would tell you to do these things:
1) Install new cables and cable housings.
2)Be sure that the cable guide underneath the bottom bracket shell is clean. Even if it is, take it out and wash and scrub it. Do not lube it as that will only attract dirt.
3) Make sure that the travel distance for the derailleur cage is adequate. You could have the high screw setting too tight, preventing the cage from moving out enough so the cable can pull it and the chain onto the 3rd chain ring.