Dave,

The 14x11 you are using is a much greater load on the engine, than is a 14x6. Yet if the engine turned the 14x6 at 10,000, it was releasing 1.49 HP, compared to the 14x11 at 7,550, which is only 1.18 HP. The Diesel is giving more torque, despite the lower calculated HP, giving 21% less HP at 24% less RPM.

I would suggest a larger prop than you are using, in the range of 15x10 to 16x12.
Also, the carburettor you are using is too large for a Diesel. A 7mm bore carburettor should fulfill this engines fuel needs.

Using too large a carburettor makes you lose power and consistency.

With a much larger prop and smaller carburettor, the Diesel will excell.