Front Mech Adjustment - Limit Screws...well, screwed?

Working on two bikes today, fairly budget mtbs, both with low end Shimano mechs, I adjust the low limit screw to get the chain centred in the thing, but shifting up to the big ring and adjusting the high limit screw seems to do nothing, in fact screwing the high limit screw only has an effect just as it's about to fully screw in, but even then the chain still fouls the cage.

Hoping to avoid the generic ''just buy a new one'' answers and wanting to actually find out what the cause of the problem could be, has anyone experienced this before, or have any ideas what it could be? Is it a double front mech that isn't compatible with the triple chainsets they're fitted to? Cable tension/barrel adjuster is screwed in all the way and the cable's in good condition. Annoying.

...and just to hijack my own post - seized pedals > help? I'm thinking I need fire...

What gear is it set to be in when you start to set it? If you start with it in the wrong gear then the first adjustment will be out and the cable won't be tight/right enough. Long shot I know.

It isn't one of them ones supposed to be cabled from above/under cabled the wrong way? An even longer shot but if someone did try to swap the mech and got the wrong type before bringing it too you.

Other than that it's seized/rusted/ruined. I hate front mechs and often with the cheaper ones you will spend more on cleaners and stuff trying to revive it than a couple of quid for a replacement.

Pedals can seize/rust, sort of thing that happens if a bike has been left outside all winter. Rain and stuff will get into the pedal bearings and ruin them, bb won't be far behind, hubs and headset also. Chain bright red too?

http://www.sportsdirect.com/muddyfox-mtb-pedals-931236?colcode=93123603 - cheap pedals
http://www.chainreactioncycles.com/shimano-un26-square-taper-bottom-bracket/rp-prod34652 - cheap bb
http://www.chainreactioncycles.com/weldtite-cycle-grease-with-teflon/rp-prod5954 - plenty of grease, hubs pedals, headsets whatever

Is it braze-on or clamp-on? If the latter, check the positioning of the whole derailleur. If the cage isn't aligned properly it's got a much higher chance of contact with the chain when it shouldn't have.
Either that or you might have it in too low a gear when testing the big ring or too high a gear when testing the small..

Other than that, I'm out of ideas

Thanks both, I've been following a few videos on't web down to a tee, and I've got the shifting working, the cables tensioned and threaded right, in lower gear for lower limit, higher ring for high limit etc (same positioning at the cassette too). But the outer limit screw and tweaking of the barrel adjuster both just have zero effect on moving the cage off the chain. They are cheaper Shimano ones like I said, so, I wanted to avoid it but I might give up and get new. Hours of adjustment has gotten me nowhere, so there's no more I can do.

It is clamp on, and I checked every positioning, up/down, side-to-side, chain cage as parallel as can be with chainring...

With the pedals, the cranks and everything else look fine, no rust to the chain, all properly greased but the pedal's just wedged in their tighter than Vanessa Feltz in a buffet exit...

One of the pedals will be reverse threaded so lefty tighty(it's so it doesn't come undone as you ride).

Re front mech adjustment, you could try a longer screw, also if the mech has been forced at anytime the cage can become bent out of alignment, sometimes it's worth gently forcing it into a more suitable position.

When I set up a front mech I always use this method;
Rear mech cable connected and tight-ish (this is just so you can shift onto the largest rear sprocket), front cable not connected at all so no tension on the front mech.
Shift rear gears onto the largest sprocket and adjust front mech inner limit screw so chain clears by a fraction.
Connect front cable to mech making sure the gear shifter is set fully over for the inner ring.
Tension front cable so you can pull it about 1-2cm at the midpoint of the downtube. Too much tension and you won't be able to shift the front mech all the way across.
Shift rear mech to a middle cog, then front mech onto outer. Now shift rear mech onto smallest.
Now your chain is as far out as it needs to be so adjust the front mech outer limit screw to bring the cage close to the chain without rubbing. Front mech cable should be pretty tight now too.
I then go on to finish off setting the rear mech up.

I have had issues before when the limit screws seem to have no effect, it was usually when the cable tension wasn't set right so the shifter couldn't move the chain all the way across.

Great stuff, thanks Krankers

Yeah, I was about to ask if you had checked the tension at the maximum shift -- looks like krankers covered it.

The Park Tool blog is a good starting point for basic spanner work.