I found out that some people were using baking powder which wasn't pure sodium bicarbonate. Only pure sodium bicarbonate and citric acid will work with this grenade. Even vinegar and lemon juice will not work.
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I have done more testing and found a better mixture for colder weather. I have tested until 15°C (59°F) and got good results with 25ml water and double the citric acid. Unfortunately I will not be able to test in colder weather because it won't get much colder where I live.
So, I suggest to use this mixture for temperatures of 15°C to 25°C (59°F to 77°F) and also use the maximum amount of baking soda possible. This should give about 3 to 4 seconds delay on the explosion. If you feel that this still gives you not good results, you could use 30ml water with double or triple the citric acid. My results were 10 out of 10 explosions.
You could also try this mixture to see how it works with temperatures of 0°C to 15°C (32°F to 59°F). If this doesn't give good results you could try up to 40ml of water and 4 times the citric acid. Although I don't really suggest using so much water because all this water mixtures does fly around and will more easily cover people and their airsoft weapons.