1985 yamaha 90 hp (90etlk), timing jumps 90 degrees

[walmekinder]

1985 yamaha 90 hp (90etlk), timing jumps 90 degrees on timing light. Mechanic is stumped. The engine engin starts up and idles well. After going over the timing, as the previous owner messed around with it. So far he has replaced the stator and pulsar, went through the fuel system, checked carbs, replaced fuel filter, added water seperator, and a new gas tank. When put in the water under load the timing was reading 24 degrees with throttle wide open.. backed it down to about 1500 rpm left it run 3 to 5 minutes then it jumped out 90degrees or 1/4 away around the fly wheel.
Anyone experience this issue or any advice. Thank you