Soak it in SeaFoam for a week. Worked wonders. I had a 25cc I WAS working on. The rings broke, and I didn't want to spend $10 on one ring, so now the parts are all over the place. However, the carburetor looked like new after the dip in SeaFoam. Like literally new, inside out.
Remove the primer however, before I dipped it in SeaFoam, it worked fine. When removed, there were cracks everywhere, and simply fell apart when pushed. Another reason why I didn't try and fix it.