The CDI ignitions supplied with the EVO gassers are designed to be run on up to a 2S lipo. Even the old ignitions that were strickly limited to 4 cell battery input could get flakey with a battery that was not holding up under load. Running these new HV ignitions on 4 cell Nimh is marginal at best.
When you get down to the 10cc size, ditching the second battery and going with an IBEC is a good move.
Pete