Originally Posted by
Jumboguy75
the chain falls off in the rear derailleur, on the 1st gear.
There should be two screws next to each other on the rear derailleur. They are called "limit" screws because they limit how far the RD can move side-to-side. One of them needs to be screwed in (clockwise) a little to keep the chain from going past the cog (rear gear). There may be an "L" and an "H" marking. The "L" (low) is for the largest cog; the "H" (high) is for the smallest. Screw the appropriate one in a little until the upper-pulley of the RD lines up under the cog.