I have a 80's 350 Volvo inboard in a jet. My problem is, it wont turn over. I have wired the starter solenoid directly through a push button. With the key in the off position, the motor turns over as expected. When I introduce the ignition circuit by turning the key on, I lose enough power that the engine will not turn over. I start the motor by cranking with the key off and then turning the key on to get it to fire. I have checked and cleaned all grounds, most all connections. I have isolated the igniton circuit from all guages, radio, etc... and it still will not turn over. I have one new battery and one battery I took to carquest, they tested it and said that it is fine. The battery leads I are not the problem as I used jumper cables to the battery switch and same result. Changed batteries out of truck and no fix. New igniton switch, battery switch, wiring, etc..... no help. I am at a total loss to as what the problem could be.