If you hear the testing buzz, that would mean that the spark occurs OUTSIDE the engine, like in the plug cap.
Take it off, and inspect well, then clean with WD40 to get rid of all moisture. After cleaning, dry well.
It may be possible that the cap was not pushed on far enough. It has happened.

2S A123 cells should work very well. If I am informed well, the low voltage before it falls off the cliff is 3.2V per cell.