On my rotary, the coils and HEI's are all powered directly with 12V. One wire feeds all 6 + terminals, by having 2 series of 3 way splits.

I would use a voltmeter to see what the input voltage to the HEI is. As stated, it should recieve full 12V to work properly. Just because you wired it with a wire that *should* be full battery voltage, doesn't mean it's actually getting there with the full amount......