10 is too low

Is the ECU wired directly to the battery?

directly to battery one. Same one I run on. Tried with using both batteries and a charger

New batterys No difference

when you made the splice in the purple wire for your ecm switched voltage ( it was a splice not a cut and connect correct ? ) it continued .........I know the answer but I had to ask,...

I had a bad connection on my ignition switch that caused that to happen to me. I had 12volts everywhere but the connection was not good enough. Same thing happened to my outdrive trim function The rivet holding the wire connector on the fuse board was loose sometimes it worked sometimes it did not. Good luck with your project

it was a soldered splice

Does the purple wire have power while cranking? I thought it dropped power while cranking....could be wrong

Did you set the timing at 50* BTDC?
If set less than that lets say 34* then during start it would spark after the compression stroke.

it has power well cranking Its tapped on the boat side not the motor side. It has battery voltage well cranking.

I dont know about the ignition timing of the top of my head. I'll be plugging in tomorrow and checking it Along with the start RPM setting.