It is normal for cheap 2-strokes to run better at no load if you make them leaner, but then they won't pull much load.
There is something you didn't tell us about your substitution of the Mustang carburetor: how did it run on the Mustang (both before and after trying it on your Vortex)?
If you establish definitely that a good carburetor will make your mower run to your personal satisfaction, we should be able to get you there with your original carburetor, but you, and we, need to know that we aren't chasing a chimaera here. Getting your mower to run much better at no load can easily be achieved just by reducing the size of the main jet or lowering the float level, but then it won't mow grass.
