Good news. I unhooked the wire from shift interrupt switch started the engine and applied 12v to the black wire. I was able to set base timing. It was way off. About 10*atdc. I adjusted to 1* btdc. Hooked everything up as it should be and cut the white looped wire. Checked timing again and it was at 12*btdc. Everything is working as it should. There is no more back firing and no miss in the engine. Will have to try the boat again before I make any assumptions about the gear ratio and prop. I'm hoping the way it was wired was my problem for rpm as the ignition couldn't advance