Bicycle Tutor

Hi, I am a newbie at bike repair but I've read a bunch of advice videos, tutorials for adjusting deraileur etc and I can't get rid of this problem:
- After bike shop replaced a broken chain on my mountain bike, I have been having shifting problems - excessive noise, slipping gears, etc... Instead of taking it back to them I want to try to figure it out myself (if for nothing else because they take forever and I don't have a lot of confidence in their abilities...)
Main problem: no matter how much I turn the low and high deraileur adjustment screws and how much I adjust the tension on the gear switch cable, the outer cage of the front deraileur always rubs against the chain when the chain is on the middle or large chain ring (for about 3/4 of rear gears). In fact, adjusting the high and low screws doesn't seem to move the chain cage much if at all. I can only move it by adjusting the gear tension knob in the shifter (or by readjusting the gear cable directly with the screw).
I haven't tried to move the deraileur itself - it might be slightly off the parallel but not by much...
Any ideas?
Also, maybe related or not - the bigger problem is that I can't pedal hard on most of my gears without "slipping" - i.e. having the rear gear slip to a lower gear on me - is that most likely a matter of fine-tuning the rear gears or something else?
Anyway, any ideas/thought much appreciated.
Thanks,
-Alex

Hi delenca,
It sounds like a problem with the H limit screw on the front derailleur. Try loosening that off all the way and increasing the cable tension a bit. It could also be that the derailleur is slightly off the parallel so you might want to try playing with that as well.

Hi Alex,
I've tried your 1st suggestion. I just didn't see much change by adjusting the high (or low) limit screws.
Adjusting the cable tension helped but at one point I increased the cable tension so much that I couldn't change gears anymore, so I've dialed it down.
I'll try your 2nd suggestion next.

For a front derailleur, you won't see much change by adjusting the high or low limit screws. The low limit screw controls (in a very fine way), where the "chain cage" is, the high limit screw controls how far it moves when shifting. So more or less, the low limit screw controls how far in the chain goes, the high limit screw controls how far out the chain goes. I'd say either you need to adjust the derailleur angle some, or more likely, increase the tension on the shifter cable by loosening it completely from the derailleur and then tightening it back on there using a pair of vise-grips (careful to not tear it up too much), or a 4th hand tool.
As for the rear gears, it sounds like the derailleur needs adjustment.

Hi Skyguy,
Thanks so much for the info. That's helpful. From my reading/watching I had the feeling the adjustment would be more dramatic but I realize now that the high/low knobs are more of a fine adjustment tool.
I agree with you. I think it's likely a combo of slight angle + tension. I haven't tried adjusting the angle yet. I did try readjusting the tension cable but I probably went way too far b/c then I couldn't shift at all! So, somewhere in between there must be a sweet spot. I am just surprised how narrow that sweet spot seems to be right now (perhaps b/c I have to tweak the angle as well).
As far as the 2nd problem, it was actually quite simple once I read the correct diagnosis elsewhere - it was a stiff chain link. Really obvious once I knew what to look for and easily fixable with a -borrowed- chain tool, as per tutorial. (http://bicycletutor.com/stiff-chain-link/).
-Alex