Trouble w/ dyi co2

Hi all

I have been trying to make a co2  setup. I am not
having any luck. It will start to bubble after a little time
then it just stops. I am using:

2liter bottle
2 cups sugar
6 cups water
1 tsp baking soda
1 package of yeast.

I have even added a valve to slow down the release.
Just can't get it to work after a couple hours. Am I
doing something wrong?? Thanks for any help.