Andy,

If your engines run well inverted, but are difficult to start inverted, well....that's common. I've had models that would run all day once started, but tended to flood too easily when inverted. Had to flip the model over to get them started.

With an electric starter, you can many times start the engine invered only if you start out with it 'dry', and allow it to draw fuel on its own. Don't prime it in that case. Just connect the glow plug battery and turn the engine over with the starter.