Water temperature plays a big part with activating yeast... Too cool and they either never wake up or are very slow to get going.  Too warm and they die.  I think the desired temp for blooming yeast was 100-110?  Just warm to the touch.  
Adding less yeast (1/4 teaspoon per 2 cup sugar, 2 cup water) allows a longer discharge of co2.  Larger amounts (1/2-1 tsp) and the yeast go crazy, have a huge party, but won't last very long.  I used the lesser yeast method in my diy.  I had 1-2bps pretty steadily for 10-12 days.  This was on a 5 & 10 gallon tank.
For the 55 I'm thinking at least 3-4 2L bottles to be effective.  Also, a warning about airstones... Co2 will breakdown the stone.  I like using a piece of bamboo chopstick - the gas passes thru the bamboo fibers and the bubbles are very fine.  If you want to splurge, a ceramic diffuser can be used with diy co2.
And I don't see a secondary water chamber on your diy setup.  By running your line into a second smaller bottle half-filled with water, so the gas bubbles thru the water and then setup another hole/tube in that cap leading to the tank (w/your check valve).  This will remove yeast spores from the co2 to help reduce "gunk" aka yeast buildup on your airstone.