Answer

This is quite odd.

 

If indeed the prop is not out issue, I haven't ruled that out yet, I am thinking that exploratory surgery is about the only method of getting to the bottom of this issue.

 

Before we start removing bolts, take a small sample of the lowerunit oil from the bottom plug. There could be a very small amount of very fine metal particles on the magnetic plug but the oil should be clean of any metal flakes and unburned.

 

Should this come out as not an issue and the oil is clean. We would test the propeller fully or simply replace it. Once sure its not the prop, we need to remove the lowerunit and start inspecting areas of possible cause. Drive shaft and crankshaft splines, gears and shift parts in lowerunit....ect.

 

One more test that I would consider is to remove the spark plugs (Did I mention Remove the spark plugs and place them on another bench). Block the prop with a 2x4 or piece of wood between the cavitation plate and the prop, now turn the flywheel with the engine in gear. Don't put much torque on the flywheel , but what we are attempting to do is test the unit as a system. If we took a pry bar or extension and used that I am sure that damage will likely occur, so be careful with this test, and it will likely tell us a lot.

 

Short answer: If the oil is clean, and considering all that you indicated in the question, I believe we still have a issue with the prop or how it is attached to the shaft.