Grip shift and rear derailleur problems

[mtnbikr]

Hey guys i have a problem, I recently got a new full suspension mountain bike for free at my local dump. It is one of those cheap wal-mart brands and it had a broken falcon rear derailleur. I got a buddy of mine to get me a new derailleur from one of his scrap bikes. I installed it and hooked it up to the cable but the darn thing just wont shift! It is stuck in 2nd gear so its a 3 speed instead of a 18 speed. Any help? I will try to answer any questions you might have. Thanks a million
-mtnbikr

--Update--
I just took it for a ride and i noticed if you try to shift (with much difficulty) you CAN shift from 2nd-6th gear. but on the lever it just moved 2 spaces. Hope this helps. Thanks
-mtnbikr

[DaveM]

First detach the cable from the derailleur. Try pulling the end of the cable gently by hand and you move the shifter up and down. If it feels like it moves smoothly, then maybe something is binding on the derailleur. If the cable doesn't move smoothly, next you have to figure out if the cable has a bad kink in it somewhere, if the cable is gummed up inside the housing it slides through, or worst case, that the shifter is messed up.

[mtnbikr]

Thanks, DaveM. I did what you said in your reply and I knoticed something was wrong with the derailleur. The spring that keeps tension on the chain does not keep tension on the chain. Thus making the chain limp when I shift on the front sprocket. I'm taking it apart to see what has happened. Will update soon. Thanks,
-mtnbikr