Problems with inverted engines are common. It's because the glowplug acts as a sump at low to mid throttle settings. The liquid fuel tends to put out the fire. Side mounting eliminates those problems.

If the engine's getting too much fuel at idle, it sounds like the tank may already be too high. With a high fuel tank position, the engine will still tend to be too rich at low to mid throttle settings, but you'll just have to live with that. Lowering the tank will help a lot, but from your description, it's not possible in your model.

The centerline of the tank should be level with the centerline of the carburetor or slightly lower.