I remember reading something Jedi wrote once, at 6v your ignition uses more current and therefore drains the battery faster so in fact a 6v pack of equivalent mah actually lasts less time than a 4.8.
Rcexl are fine, just use the correct voltage and mount them with some kind of cushion so it doesn't vibrate to bits.
I saw one fellow who cable tied it directly to the stand offs close to the muffler, when I asked if that was a good idea he snapped at me, so I dropped it.
His ignition will fail and he'll blame rcexl for it being crap.

Back to the topic, take out the regulator and try that first, check the voltage on your battery, 4.8v 2000mah is more than enough.
Your 6v supply may have already fried the ignition though.
If that fails, pull the jug off and look at the ring, a repeated "ta ta ta"sound coupled with low compression leads to that.
You said it had low compression, did it just start or was it always like that?