One major thing I can think of that could cause this problem and is a simple fix is, the gasket or gaskets between the carburetor or reed valve and engine block could be sucking air. I also bought a Chinese engine, the first thing I did was to replace the carburetor, and put a Bowman ring in mine. The jury is still out on whether or not I replace the CDI.

When I bought a Bowman ring, Frank Bowman inserts a notice into his packages that tells you that the Reed valves on these Chinese engines are notorious for having mold flash left on them which prevents the Carburetor or engine block side from sealing. You might give it a check before you throw in the towel. Good luck