By: Manic Bang
 
I used the soda bottle method, and in 1 week, it used to stop producing CO2.
I suspected lack of airtight seal was causing this.
 
I have found a better method - with 2 cups sugar and 2 liter water and 1/2 tsp yeast, it consistently makes CO2 for 3-4 weeks.
 
What I use is a Heinz Ketchup bottle (64 oz). It already comes with a hole in the top (ketchup nozzle). I found that the front part of a BIC ball pen fits very snugly in the ketchup top (see pictures), and as the front of ball pen is conical, you can get a very tight fit using airline tubing.
 
This method has really worked great for me, and has been the best DIY CO2 generator I have been able to come up with (believe me, I have tried a lot of methods, soda bottle, Apple juice bottles, glass bottles).
 
Also, please clean your bottles thoroughly before adding the next charge of sugar solution and yeast (yeast and lactobacillI bacteria are found in air, and presence of bacteria undermines the ability of the yeast).