I fly this engine all the time on different planes. Some with fairly enclosed/restricted cowls...


The prop you are using is your most likely problem.

Use a smaller higher pitch prop.

Get a temperature guage and measure the head temp just off the glow plug.

Run the engine at full throttle for 3 minutes on the ground and immediately measure the temp.

If it's over 340 degrees F, richen the engine settings until the engine stays around 310 or lower.