None of the big YS engines are what you would call smooth. They all vibrate a bit. Upright or invert doesn't make a difference.
RPM drop when you take the clip off indicates too rich or poor quality fuel. You may have to go bigger on the drill size on the air bleed.
The low end is correct when it will continue to run without leaning out or richening at idle. If it takes the throttle ok it is close.
If you think it is running hot, reduce the load with a lighter prop. You can run these at 9000 with no problem.