I bought a 1987 Chaparral 20' boat with the Mercruiser 230, V8, 5.0L Thunderbolt IV ignition system. I am at my wits end chasing a starting issue. Put on a new carburetor, ignition coil, cap, rotor, and spark plug wires. Sometimes it starts, sometimes it doesn't. Been checking the voltage at the + positive side of the coil and get around 8v with key on. (No start), Other times when I turn the key on, I get almost 12v (11.5v) and the boat will start. All the wires are connected and cleaned. What is causing the constant change in voltage to the coil and the no start? I cannot figure out why the voltage drops?