[sony2000]

Sounds like a turbo problem. Check that the hoses are secure and not leaking, as well as the wastegate is not partially open or opening before getting to the correct boost pressure..

 

[sony2000]

If the bearings of the turbo were starved of oil, from lack of oil or shutting down the engine to soon after work outs, then it can't get its rpm, and would be kaput.

 

[sony2000]

Knowing the history of the head, check compression. Secondly Yanmars have seating of the valves problem,occasionally.
Over boosting is a cause of blowing a head gasket. So a wastegate malfunction may still be not resolved.

 

[sony2000]

The prop became overpitched and the 125hp couldn't rev up past 1500 rpm.
Always check the cheapest repairs first (prop), before the ideas get too grand (turbo) and expensive.