RE: Voltage Converter?
Is this a kero start?
The Castle regulator is rated for 10 amps max. An ECU battery needs to be able to supply a fair amount of amps during start up. Kero start a lot more Amps than gas start.
You might want to find a Nixx pack to borrow to do a test to see how many Amps your setup needs to start, then you will know what you need to do on the power supply side.
A 6 cell 7.2V NiCd or NiMh fully charged is near 9V, a 2 cell 7.4V LiPo fully charged is 8.4V.
If you have wires getting hot, I would do some checking to be sure that there is not some other issue.