The sound you describe makes me think of a seize happen to the cylinder. And what is known as a soft seize if the piston is still able to move inside the cylinder. However the rings may have broken or been fused to the piston which wont let them expand to make a seal on the cylinder.
The cause of the seize is most likely an air leak around the carb when you put it back on after cleaning it. That or an air leak may have developed at the head someplace.
Take the cylinder off and inspect the walls to make sure they are still smooth and not pitted or gouged. Inspect the piston and rings. If everything actually looks and feels fine them reassemble and check for air leaks before you head out on a ride. You may need to replace a ring, the piston, or possibly the whole cylinder if the damage is bad tho.
Post findings and any additional questions. See the wiki on here for garelli engine info. There is a repair manual for your engine on there for sure.