Send my a PM with your email
address and I will send you an article that I wrote on curing my Yanmar
start problem. My problem was traced to the wiring
harness connectors between the control panel
and the starter solenoid. I disconnected the key switch wires in the harness and replaced them with 10AWG wires--straight run--no splices between the key switch and the starter solenoid. Problem cured, permanently.
Some Yanmar engines have an interposing relay in the wiring harness (used on very long harness extensions.) Most boat owners are unaware of their existence and unfortunately, many mechanics overlook them also. I have sat through some very long and sad tales about this one.