I recently broke in a mag .28 a little more than 1/4 inverted. The things I found useful to do are:

1. prime engine inverted but before moving finger flip the engine up right to get the fuel into the shaft. If you don't you will see the fuel dripping from the carb.

2. when refueling don't turn the engine up right, the fuel will fill the muffler quicker than a thief leaving a bank after a stick up.

3.make sure once you have the engine running it is running counter clock wise. I have never had an engine run clock wise until this inverted mess was done.

4. try different plugs , no explanation except when I switch from an OS A3 to an OS 8 the engine ran better.

5. Although inverted the up down ground test is the same .

6. good luck I hope this helps.