These wires are often a problem with yanmar engines after a few years. There are spade connectors and fuses in these lines and they tend to corrode. After a while there is not sufficient voltage to fully engage the solenoid and it chatters or just clicks and then slowly burns the connections in the solenoid.

The best thing is to just run new wires and make them 10 gauge or so and put a fuse in the line. You should run a larger wire up to the panel from the engine also.

Total, you need:

large ground wire to block
large hot wire to starter
small solenoid wire to the starter button.
sometimes there is a small ground wire to the solenoid also.

Also there is a fused power line from the engine to the panel.