If you detach your shifter cable it should default into the smallest cog. If it's not going into the smallest cog with the cable detached then check your limit screw.
Generally when I'm adjusting my derailleur, I'll follow this procedure
- Detach Cable
- Adjust limit screw so it runs properly on smallest cog.
- Put barrel adjuster almost all the way in, with a small amount of room to loosen it.
- Re-attach cable making sure it's reasonable tight, but not so tight that it will shift info second smallest cog, making sure cable outers are properly seated in all cable stops.
- Shift into second smallest cog and if it doesn't make it then use barrel adjuster until it has enough tension to get into second smallest gear.
- Shift back into smallest gear to make sure it goes there, using barrel adjuster if necessary.
- Go through all gears on cassette and do fine adjustments with barrel adjuster to make sure all the gears are working properly.
- Check the limit screw for the biggest cog to make sure you can't shift into the wheel. You should try pushing with your hand as indexed shifters often won't shift past the last cog. You can do a visual inspection on the limit screws to ensure they are touching against the stop when the derailleur is in the biggest and smallest cogs.
A nice tip I learned for dealing with the barrel adjuster is to turn it in the direction you want the chain to go. For instance, if the chain doesn't go to the bigger cog when it is supposed to, turn the barrel adjuster towards the wheel. If it's not going down to a smaller cog, then turn it away from the wheel until it makes the necessary shift.
