My charts indicate 10 X 6 for a 40 engine. Some people will run bigger props after break in. For break in and trouble shooting I prefer the engine run freely and not under heavy load. In addition heaver loads aggravate the problem that you describe. All of my 45’s fly epoxy heavy airplanes better with a 10 X 6 prop.

After the problem is solved a prop can be selected to fit your flying style and airplane characteristics.

Experience getting off the ground too slow has never been pretty for me so it is important for the engine running properly.

You will find the problem.

Bill