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I have done this a few times and I did two things different.1st I put a simple valve at the bottle that holds the drink. when the pressure is equal and the max is reached I close off the valve locking in the gas. next the soda mixture is cold next to freezing..a few floating ice chunks is good. lastly with the valve shut I shake the bottle vigorously for a minute or so. the more surgace area exposed to the co2 the more will be absorbed. I let it rest in the fridge for a while before pouring a cold tall glass of bubbly lemon goodness!