co2 generator

Question
QUESTION: Dave.I have had  co2 generators  for some time but am getting tired of changing it out every 7 days or so.  This is my system,   2 litre bottle with 2 cups of sugar, warm or tepid water, shaken  until sugar dissolves,  then 1 1/2 tsp of yeast flieschmanns.  I have tried bringing the yeast to room temp then mixing with a tble spoon or two of water shaking it up every few minutes until it is good and dissolved then adding it to the sugar mixture.  It works for a few days then slowly dies out.  I have read and talked to other people about this and they seem to think that this system should last 10-14 days.  Sugar isn't that expensive but it is just a hassle going thru it every few days.  Am I doing something wrong or just what?
It doesn't matter I am sure but I have a 100 gallon tank and have two of these set up, alternating bottles as they go dead.

ANSWER: It should (the yeast)live for until the sugar runs out... How warm is the tank??? dave

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QUESTION: tank temp is steady at 77 degrees  and has been since the tank has been set up.  since original question I have had to replenish the co2 generator.          kevin

ANSWER: Have you tried to adjust the temperature to about 80-82 degrees????  dave

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QUESTION: Dave, just out of curiosity what does the temp of the tank water have to do with the co2 generator since it is outside of the tank.

Answer
Yeast is very sensitive to heat and has a very exact temperature range. If the tank is too warm or the room is too warm or cold then the yeast will not reproduce and thus you will not have very much CO2... Tanks will if too cold suck the warmth out of a room... the the temperature of the tank is important.  dave