What plug are you using? You won't need a different plug to run the engine inverted. All that is required is the tank to be placed properly and the engine tuned.

You say you lowered the tank, but don't think that will resolve the problem completely. Have you actually run the engine yet?

Some people preach using a Cline Regulator on inverted engines. I have never needed to go that approach, but your findings may be different.

John