It sounds like you are running a bit rich, and the engine is flooding. Inverted engines take a little doing.

Don't prime quite as much as you are when getting ready to start it. You are basically filling the cylinder with liquid fuel, and that's causing a hydrolic lock, and yes, you can really break things this way if you aren't careful.

If you can rotate the engine even a little off totally upside down, it might help.

You might also try an idle bar glow plug, or a hotter plug, it might help the idle and low speed running.