I did run into a problem a while back, where I tried a couple of Life batteries on a MVVS engine and it would just cough, sputter, kick back, and in general misbehave terribly. At the time I thought i had some other problem such as a bad ignition module or timing sensor. But later I used a 4.8v NMH pack and the engine ran perfectly. So I tried the original ignition module that was giving me all the troubles and it worked perfectly too. That was when I discovered that not all LiFe battery packs may work with the ignition units. Subsequently I had bought a Mag Dog RC LiFe two cell pack and it worked great with the ignition modules too. So I have been sticking to using Mad Dog Rc liFe packs when I needed receiver or ignition module battery packs. None of them has disappointed me yet. But those LiFe packs I bought cheap to use for ignition modules just don't provide enough current to work for ignitions or servos for that matter.

Another thought is to make sure your ignition module will handle 6.6v from a two cell liFe pack. Many of the older units were not deigned for more than 6.0v maximum, and a 5 cell Nicad or NMH pack or a two cell LiFe pack will exceed that 6.0v maximum when fully charged.

I did get a CH Ignitions conversion for my MVVS 58cc gas engine and it works great too. The old two magnet Valach ignition modules were very sophisticated but prone to oddball failure or weird problems. Going to a CH Ignitions setup worked great.