Can't get my 1991 4.3 v6 Yamaha boat motor to start. The boat is a celebrity cx 190. It's getting fuel, and starts the starter, but after checking the voltage of all the wires we narrowed it down to the distributor cap isn't getting any spark. I have replaced the coil, an all new distributor assembly, new cap and rotor, new battery and it still won't fire. My navigation lights, trim , blower, bilge and even radio all have full power . All the basic things to check on a motor for spark we've checked. Is there anything else I'm missing?? Any suggestions?