carbonation issue with soda

im having carbonation problem with store bought soda . so, i wanted to try despensing soda out of my keg. so i bought 2 bottles of 2L diet sierra mist and put it in my keg just to try to dipense it intially and this worked fine. after 2 day the soda poured out fine at 12 psi but is kind of flat as soon as the initial bubble go away(about 30 secounds).so.... i raised the psi to 28 psi for a day then lowered it to 12psi to dipense and see what would happen. now the soda comes out with alot of foam and looks super carbonated but after 30 sec it start to go flat again. why does is keep going flat right after served??..........WHAT AM I DOING WRONG???????

What's your beer/soda line hose length?

Soda has a much higher carbonation level than beer. At 40 degrees, the psi on the soda should be about 30 psi. To dispense, you need super long lines (to avoid "knocking" the co2 out of suspension when being served). I have 25' lines to dispense soda, but I noticed that 30' is a bit better.

wow 30' of line....well that explains it, i only have 5' of 3/16 id line.so will this happen with beer also? will 5' of line knock the carbonation out of the beer also? i just got this system about 2 weeks ago and haven't started despensing beer yet (my first time brewing)

Generally speaking, yes. 5 feet is going to be too short. Its something about the line's reistance per foot x some factor to get the psi, or was it the reverse...
Anyways, I think it's usually 6-10 feet for beer, and 15-30 for soda. Something about the psi/2 as a rough resistance estimate.

Best estimate is to get a longer line. You can always shorten a line if you have too much of it. My beer line is 15 feet. If I do soda, I'm going to take Yoop's recommendation and go for 30 feet or maybe the mixing sticks. (Right now, I just carbonate in 2 liter bottles, but now that I have my acid phosphate, I want to do more soda mixtures.)

Yooper is right......when I changed lines it made a great change in my carb levels. I used about 25 feet of 3/16 id line. It made a world of difference. I almost look like I know what Im doing now.

Keep in mind that soda and beer are carbed totally different amounts! Soda is typically at 30 psi while beer may be at 12 psi at the same temperature. So, you'll need different regulators to keep those separate pressures, as well as different length serving lines.