I've got a Hagen Co2 Unit, and followed Anona's recipe until recently.
It never lasted me very long (1 week), so I thought I'd experiment.
I boiled 8 cups of water, and gradually added sugar cup by cup.
I managed to pack 6 cups of sugar in there, and then let about
half a cup of water boil off. After cooling to room temp, I put
the sugary water (more like syrup) into my hagen unit, added the
recommended dose of yeast and baking soda and....
Nothing. I think the consistancy of the sugar-water didn't allow
for the yeast to "mingle with it". Lesson learned, I guess.
On a second note though, I diluted the mixture some (1:1), and it
seems to be producing fine. Not very strong though. I'll be sure to do
some experimenting tonight.