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Hi and thank you for your question.
I suggest inspecting the pump tunnel and impeller. You are looking for any signs of physical damage or play in the drive shaft bearings.
Since the engine has good compression, it leads me to believe the problem is in the pump or drive line for it. If need be, you can take the spark plugs out, the cover off the coupler and turn the crank/drive line by hand to see if you feel the same grittiness while turning it that you hear when running it.
Let me know what you find.
Just following up on this issue. Have you made any headway on it or is there any additional questions I can answer for you? If so, let me know.