I converted one the Craftsman 32 cc engines I have. It is simmilar to a Ryobi. I like the layout. I was thinking of making a WAL seaplane with two of these engines. This engine runs nice and smooth and hand sarts easily.
Unfortunately, it is a dog. 18x10 prop is getting only 6400 rpm. Really small props like 15x8 don't gain the rpms that they should, in my opinion.

I am using a carb intended for a DLE 30. I kept the flywheel but turned off most of the cooling fins. I bent the reed valve stop up a little. Removing the muffler only gains 200 RPM. The exhaust port is open for 126-degrees of rotation.

Should I open up the top of the exhaust port a little?

Is there a design feature/flaw of these engines that prevents decent power being made?